Rivers, river systems, watersheds
Water coming to the surface of the earth in the form of precipitation or outputs underground water, flows under the action of gravity in the direction of lowering the area, forms surface watercourses. The water is first assembled into separate streams, then in streams, and the last, gradually connecting, form river. The river is an aqueous stream flowing in the channel formed and having its own catchment.
The system is constantly and temporarily active watercourses, lakes, swamps, reservoirs form hydrographic network.The river network is part of hydrographic network. Modern river network was created for many millennia. Throughout the geological epoch, it has undergone changes due to tectonic processes (raising and lowering land crust sites), the advantage and retreat of glaciers, transgression and the regression of the sea, as well as as a result of erosion activities themselves water streams and anthropogenic activity.
The main river flowing into the receiving reservoir (sea or lake) and all tributaries that collect water from the catchment area make up river system.
There are inflows of various orders. Rivers flowing directly in main river, are called first-order tributaries, the tributaries of these tributaries is order, etc. American hydrologist Horton offered another classification of tributaries. He calls the first-order river or an elementary river, a river that does not have tributaries, the river of the second order - the river taking the tributaries of only first order, etc. Thus, the greater the number of the main river, especially complex nature Wears the river system of this river.
River system characterized the length of the river, them curativeness and dense a river network. Under long, the total length of all rivers constituting this system is understood. River length is usually measured by maps possible greater scale (in the 60 years in the USSR, such calculations were made on cards 1: 100,000 scale).
Rivers are always to a greater or lesser degree of winding. Torture is determined by the terrain, which flows the river, degree of privacy mountain breeds, Forts of the valley and coast of the channel erosion, as well as the dynamic properties of the stream itself. The resilience of the river on the site is characterized by the ratio of the length of the river on the site under consideration to the length of the straight line, connecting the beginning and end of the site (or from sources to the mouth). This ratio is called the coefficient of torture.
KIZ \u003d River Length / Length Direct
The degree of saturation of the territory by watercourses (drainage) is characterized by a river network thickness ratio. It is equal to the ratio of the total length of all watercourses (σL) of all watercourses to the area of \u200b\u200bthe catchment (F):
D \u003d σl / f km / km 2
The thickness of the river network depends on the relief, the geological structure of the terrain, the properties of the soil, climate, etc. In the north, it is usually more than in the south, in the mountains more than on the plain (however, some areas of the mountains can be arid -chuish steppe, and a small density ).
So, on the plains of the Pre-Boal D \u003d 0.05 km / km 2, and on the northern slopes of the main Caucasian ridge-1.5 km / km 2. In Belarus he fluctuates in natural conditions In the range of 0.2-0.5 km / km 2, but taking into account the conduct of the drainage drainage reaches 1-5 km / km 2.
The concept of the length of the sloping flow of precipitation is associated with the river nettime ratio. The less often the river network, the greater the path should be done on the surface of the slope to the bed. The average slope length is defined as:
DSC \u003d 1 / 2D km.
Line on ground surfaceseparating the flow of atmospheric precipitation by two oppositely directed slopes is called watershed.
All earth Parts are divided into two main subtlecies in which water flows from continents in the world ocean: Atlantic and Pacific - Indian. The watershed between these two samples is called world watershed.
World watershed or the main watershed of the Earth extends from Cape Gorn in the Far South South America According to Andam and Cordillera to Bering Strait. In Northeast, Eurasia, he enters the limits of Asia and passes through the Chukchi ridge, Anadyr plateau, Mountain Ranges Gydan, Jugjur, Bed, Apple Table, then crosses Central Asia, Northern part of the Arabian Peninsula and enters Africa. Here it extends almost meridional direction, approaching in the eastern part of the mainland to the Indian Ocean.
Lines on the earth's surface, separating land areas, the drain from which is directed to various oceans and the sea, are called watersheds of the oceans and seas.
Watersheds separating parts of sushi, the drain from which is directed to those or other river systems, call river watersheds or river swimming pool waters.
River- It is watercourse that has a course in the continuation of the most part of the year, receiving food from its catchment and having a well-pronounced channel formed by the water lodge. Spring, giving rise to the river, or the yield of the river flow from the lake, swamps, glacier - source rivers. Place (target) of the river falling into another river or reservoir (sea, lake) - mouth rivers. On the large rivers Allocate the upper, average and lower flow.
The main morphometric characteristics (form parameters) of the river as a whole - its length and area of \u200b\u200bthe catchment area (pool).
Water civilian river - part of the earth's surface and strata of soils and soils, from where this river Gets its food. River pool - This is a part of the sushi, which flows this river with all its tributaries, including temporary watercourses, and a limited watershed. Selfless territory inside the pool in the catchment is not included. In areas of sufficient moisture, the catchment and the pool, as a rule, coincide. ( The faceless area is part of the sushi that has no connection through the river systems with the oceans.)
The combination of consistently merging streams, rivers and rivers forming increasingly large watercourses are called river system(River Network). IN river system You can select the main river flowing into the sea or faceless lake, and the sequence of tributaries of various orders.
The density of the river network is defined as the ratio of the length of the rivers l. On the square to this area (pool area) F.: 
.
By pool area Rivers subdivide:
big - F \u003e.50,000 km;
average - F.\u003d 2000-50000 km;
small - F.<2000 км .
A large river usually crosses two or more natural zones, the hydrological mode of the middle river reflects the conditions of one zone or subzones, the mode of small rivers is largely determined by local conditions.
By lenght (L.) Usually rivers are usually a river from 10 to 100 km long (sometimes up to 200), the river up to 10 km long is often called streams.
Distinguish physico-geographical and morphometric characteristics of pools. First include:
Geographical position (geographical coordinates, proximity to seas, deserts, mountain ranges);
Climatic conditions (atmospheric precipitation, temperature, air humidity deficiency);
(Humidity deficit - the difference between the pressure of saturated steam and the pressure of steam[PA]that is, between the maximum and absolute humidity of the air[g / m³] . The absolute humidity of the air is the mass of the water vapor contained in the air volume unit, that is, the density of the water contained in the air,[g / m³] ; In the atmosphere ranges from 0.1-1.0 g / m³( in winter over the continents) up to 30 g / m³ and more(in the Equatorial zone). Maximum air humidity(border of saturation) - the amount of water vapor, which may be contained in the air at a certain temperature in thermodynamic equilibrium(maximum air humidity value at a given temperature),[g / m³]. With an increase in air temperature, its maximum humidity increases.)
Geological structure and soil cover (fracturing of rocks, karst phenomena, mechanical composition of soils, water permeability, etc.);
Relief of the catchment (the slopes of the surface of the Earth, affecting the flow rate of water);
Vegetation cover (types of vegetation);
Freezing of the soils (geographical distribution of permanent, the layer of seasonal freezing, the thickness of the eternal permafrost);
Departure degree expressed by the coefficient of deposit - the attitude of forest areas to the pool area 
;
Lakery of the pool, expressed by the coefficient of lake - 
where - the area of \u200b\u200bthe mirror lakes;
Swimming pool, expressed by a wetlastom coefficient - 
.
TO morphometric characteristics The pools include the parameters of the river waterboat form (pool): area, length, the largest and average width, the average height, the average slope of the surface, asymmetry coefficient.
River valleys - This is a relatively narrow deepening of the earth's surface, formed as a result of tectonic deformations and activities of glaciers, as well as subsequent formation under the action of continuously flowing water.
River Valley Elements:
road - the lowest part of the valley engaged in the river in small periods of the year;
foama - part of the valley poured at the highest water level;
inappropriate terraces - relatively flat plots of valleys, which are remnants of understanding the preceding stages of the valley's development;
indigenous shores- The slopes of the valley above the highest terrace.
Rods and floodplains form bottom valley, terraces and indigenous shores - the slopes of the valley. Oldfloors, terraces, indigenous shores - excess of their rods above the water level in a malfunction period of the year.
The main difference between the river bed and the floodplain is that the boundaries of the river bed are clearly defined by the shores and rods of the bed. The floodplains does not have such clear boundaries on the slopes of the valley, since the height of the floods and floods is constantly changing.
Types of valleyson genesis - tectonic, glacial, erosion; by form of transverse profile - Canyons, Gorges, V-shaped, trough-shaped (tropows), trapezoidal, box-shaped.
Longitudinal profile of the river - schedule for changing the marks of the water surface and the bottom along the length of the river.
Fall of the river - The difference in the marks of the water surface or bottom () on which either area of \u200b\u200bthe river. Full fall - The difference in the marks of the water surface or the bottom between the source and the mouth of the river. Bias rivers ( I.) - The ratio of the fall of the river on the site to its length is expressed in the shares of a unit or ppm (‰). For medium in size of plain rivers, as a rule, I.<1‰, для горных – до нескольких десятков ‰.
Types of longitudinal profile: Went, straight, convex, stepped.
Main basis erosion Rivers - level of receiving reservoir or watercourse.
Along the valley of the main river in each river system can be allocated three characteristic zones (Figure 8):
I - the erosion zone with the greatest longitudinal slopes (rover of rivers) is characterized by a systematic removal of the nanos with a gradual insert of the bottom of the bed in indigenous breed;
Figure 8 - Characteristic zones (I-III) Valley of the main river
II - the transit zone on average rivers is characterized by constancy of the slopes and medium heights (marks) of the bottom of the bed;
III - the accumulation zone (the mouth of rivers) is distinguished by the redundant injection of the nanos, providing systematic extension of the heights of the bottom.
In the erosion zone, under the action of consumption, the river river in indigenous rocks occurs with the appropriate removal of the hosts to the underlying areas. The bottom slowly decreases with the corresponding decrease in the slopes. Cutting ends where the bias of the valley is so small that the river can no longer wash the indigenous rocks.
In the transit zone, the biases and heights of the bottom do not change, the nanos is moving. In connection with the increase in the area of \u200b\u200bthe catchment, consumption is growing 
, and the bias of the river turns out to be less than in the erosion zone: 
.
In the accumulation zone, the river stream is unable to transfer all applicants from above, so the river, gradually increasing the height of the bottom and water levels, seeks to create an increased slope. Water consumption may increase due to the growth of the area of \u200b\u200bthe catchment, but may decrease 
Due to the lack of tributaries on the mouth and flooding flood.
The mouths of the main rivers are different types (Figure 9).
Delta - The mouth of the rivers flowing into the drawless lakes, or the seas with minor tidal and taming phenomena, for example, p. Volga.
Lip - The mouth of the rivers flowing into the tidal and tump and oceans, for example, p. Oka. The lip is continuously growing upstream due to the erods of ice occurring due to sharp differences. In the sea (ocean) opposite the lip, the island is usually made, folded by products of the river bed during the formation of the lip.
Liman - The mouth of rivers formed as a result of geological catastrophe, for example, Dnipro and Bug rivers.
a - Delta; b - lip; in - Liman.
Figure 9 - Characteristic mouth of major rivers
Rivers behave differently within each characteristic zone and form valleys of a different structure, which should be taken into account when designing, for example, bridge transitions (Figure 10).
In the zone of the erosion of the river proceed by narrow valleys - canyons. In this case, there is an irreversible cutting of the bed into modern alluvius or indigenous rocks with the corresponding removal of the products of the erosion on the underlying areas of the river and a decrease in longitudinal slopes. The river valleys in the erosion zone are relatively narrow, they are characterized by a high occurrence of indigenous rocks and a small thickness of alluvial sediments (drone-forming nansions). The rate of irreversible decrease in the bottom of the river (and respectively water levels) 
Estimated according to the results of measurements of the heights (marks) of the bottom or water levels for different years.
Then the total irreversible decrease 
For the estimated service life of the bridge, which must be taken into account when the poundings of the supports will be equal to:
,
where - the measured lowering of the bottom (or water levels) for the period T. years.
and - in the erosion zone; b - in the transit zone; in - in the accumulation zone;
1 - indigenous breeds; 2 - ancient alluvium; 3 - Naoles floodplains;
4 - modern alluvium; 5 - Fombings
Figure 4 - the structure of river valleys in different characteristic zones
In the transit zone, the nanos, brought from the upper river, are fully transported by the stream to the underlying areas. At the same time, the biases and dimensions of the river bed remain unchanged for decades. The valleys of rivers in the transit zone are characterized, as a rule, a large width, a significant thickness of alluvial deposits and a relatively deep occurrence of non-blurred indigenous rocks. Immediately above the indigenous rocks, an ancient alluvium occurs, which formed in the early stages (in an ancient period) of the formation of the river. Above the thickness of the incoherent modern alluvia, periodically tolerated by the river stream. Even above, the connected soils of the rally floodplains.
The quantitative characteristic of the river network includes its density. Dense a river network It is called the length of the river network, per unit area of \u200b\u200bany territory. You can count the luxury of the river network within the river basin. Then we get a luxury luxury for this river system: 
,
where - the length of all watercourses (km) on the area under consideration F. (km).
The density of the river network increases with the growth of atmospheric precipitation and decreases with an increase in the water permeability of the soil, wetlastics and damage to the territory. The root system of plants contributes to the water permeability of the soil.
The influence of the swattime of the river network is expressed by the dependence of N.N. Zakharovskaya:
,
where - the density of the river network of the pool;
The average density of the river network of the region, found by a large number of pools included in this region, including unbounded pools;
Wetlability coefficient;
e. - the basis of natural logarithms;
A, B, to - parameters of the equation.
In the plain conditions of the bed of almost all rivers (except for individual large) in terms of winding. The degree of winding the bed is characterized the coefficient of torture of the river:
,
where L. - the length of the river on the site under consideration;
Length straight from beginning to end of the site.
The length of the river is usually counted from the source downstream along the largest depth. The position of this line is determined by the Farvater, according to which shipping is carried out.
Farvater, passing by the main river sleeves, are called the main fairway. The line with the lowest flooring of the river bed is called talvegom rivers.
A plot of a winding river channel between two adjacent points of the inflection of its axial line is called empty rivers.
A form of a bottom relief form characteristic of the low-altar rivers, and usually in the form of a wide ridge, crossing the channel at an angle to the general direction of the flow, which causes its deviation from one shore to another, is called perekat.
The deep-water portion of the river, which is between the rods, is called pleas.
River and their regime
It is called a permanent aqueous stream that flows through the part developed by it in the decrees of the earth's surface. The precipitate falls on the surface of the sushi flow across the slopes in small recesses, forming a stream. Brooks, connecting with each other, turn into small rivers, which, continuing to merge together, form larger rivers. The described process of formation of rivers is not the only one. Rivers can flow from lakes or swamps; appear as a result of melting glaciers and eternal snow in highland areas; Arriving from springs feedable with underground waters.
Valley and river bed. It is called elongated in the earth's surface elongated in the bottom of the geological processes and centuries-old activities of water flows.
Depending on the relief of the terrain, geological origin and the nature of the soils, of which the slopes and bottom of the valleys are composed, they may have a different form in cross section: from a slight-shaped with sheer slopes occurring in mountainous areas, to widely unclear expressed recesses with very gentle slopes, merging with surrounding plain terrain. In mountainous areas there are very deep river valleys (up to 2000 m), and in the plain - the depth of the valley does not exceed 200-300 m.
In the form of a transverse profile, river valleys are divided into close, gorge, canyons, V-shaped, trapezoidal, box-shaped, troughting, etc. in the transverse profile of the valley (Fig. 6.3, a)
the slopes of the valleys are distinguished (together with the ledge of the valley and inappropriate terraces) and the bottom of the valley.
The lowest part of the valley, on which water flows, is called a r y with l about m and. There are meters from (the main) course, through which the flow of water takes place all year, and p about y y y r y with l and, slept only during periods of floods.
The pairing lines of the banks of the cross-line with the floodplain are called B R O V to A M and, and the lines of intersection of the water surface with the shores - the r e z and m and in about d. The line passing along the stream along the lowest points of the bottom of the bed, is called and or d and n and m and and or d and m and m and and or d and n and m and k and or d about about with The line, passing along the river in the midst of the width of the channel, is called a g e o m o z o p o t o k a.
The source and mouth of the river. The source is called the place from where the river originates. On rivers arising from the lakes, a river intersection point with a lake circuit is taken for the source; On marsh rivers - a place where a pronounced course begins to appear with a noticeable course. Often for the source of the river, the confluence of the two of her tributaries with different names is accepted. For example, the OB River is formed from the confluence of the Bii and Katun rivers.
The location of the river in the lake, the sea or another river is called with the river. If the river is branched onto the mouth of several sleeves, the mouth of the largest sleeves is taken for her mouth. In arid areas, some rivers do not have mouths. They lose their waters to evaporate and leakage into the ground, not reaching the sea, lake or another river.
Length and winding river. D l and N about th th is called the distance between the mouth and the source, measured taking into account all its convolutions along the river straggle.
To assess the degree of emission, the river should be divided into a number of more or less large areas, within which the general direction of the flow is preserved (Fig. 8.1, a). The emission of each site is characterized by the e-f f and c and e n t o m and z in and l and with t and, which is equal to the ratio of the true length of the site to the length of the straight line connecting its ends. For plots 1-2, 2-3 and 3-4, the coefficients of torture are determined by the relationship:
The weighted average of the coefficient of the coefficient of the river between points 1 and 4 can be found by the formula
. (8.8)
River channels branched onto the sleeves (Fig. 8.1, b). Measuring the length of the river in this case should be conducted by the main channel for which the fastest sleeve is taken.
Fig.8.1. Tortureness and branching of rivers:
a - to determine the coefficient of the uptime of the river;
b - multicast river bed
The degree of branching of the river is characterized by the ratio of the total length of all hoses, including the main line, to the length of the latter. This relationship is called about E F F and C and E N T O M R A C E T V L E N O S T and.
The river beds in the form in terms are divided into straight-line, winding (minelarizing), separated on the sleeves, scattered (wandering) (Fig. 6.4).
Fall and bias of the river. The fall of the river is the difference in the levels of water levels in its origin and mouth, and the slope is the attitude of the fall to the length of the river. In the same way, drops and slopes are determined for individual sections of the river. The biases of the rivers are expressed in the form of decimal fractions or in a community (thousandths). For example, the average bias of the northern dock I \u003d 0.00007 \u003d 0.07.
Sometimes, for the characteristics of the river, the concept is used by a kilometer drop (the drop in the water level in centimeters per 1 km of the river length).
The biases of the rivers depend primarily from the terrain of the terrain on which they proceed. In the plain areas, the slopes of rivers are very small (several decimal), and in the mountainous area they reach several hundredths. Large watercourses have, as a rule, smaller slopes than small rivers flowing into the same geographic areas.
The river bonus varies along the flow, decreasing most often from the source to the mouth. The slopes of individual sections of the river depend on the relief of the bottom and the planned outline of the bed. The bias of the river varies in time with a change in water level.
The longitudinal profile of the river is a graph of the bottom of the bottom and the water surface along the channel. On the horizontal axis of the graph lay the distance along the length of the river, on the vertical - absolute or conditional levels of the bottom (usually along the largest depth) and water level. For longitudinal distances and heights usually take different scales.
The difference in the marks of the bottom or the water surface of the river on some of its plot is called a drop (ΔH). The difference in the origins of the source and the mouth of the river is a complete drop of river.
Due to the fact that the depth rivers are usually incommensurable less complete fall, the graphs of changing the marks of the bottom and the water surface for the entire river merge into one line.
Longitudinal profiles of rivers can be planned, straight, convex, steps (Fig. 6.5, a). The nature of the longitudinal profile is determined by the geological structure and relief of the river basin, as well as the erosion-accumulative activity of the stream itself.
The excerpts of the longitudinal profile are usually confined to the places of fallacies (below their profile, as a rule, is broken), as well as local erosion basisms, which can be the main river for influx, thresholds, waterfalls, flow lakes, reservoirs, etc. The reservoir (ocean, the sea, a blindfold lake), where the river flows, is called the common basis of erosion.
The longitudinal profile of the river on the short section is very characteristic, which includes Ples and rigs (Fig. 6.5, b). In this case, the longitudinal profile is built separately for the bottom and the water surface of the river. From data fig. 6.5, it can be seen how the longitudinal profile of the water surface changes with a change in the water level in the river. At low levels (in the interleum), the longitudinal profile of the water surface is sharper on the rigs and more gently on the Ples. At high levels (in flood), the longitudinal profile is usually aligned or even becomes more steep than on the rolling.
River systems. The river system is called a totality of aquatic streams, flowing into one main river carrying their waters in the ocean, sea or lake. Rivers flowing directly to the main river are called first-order tributaries. In the first-order tributaries, rivers flow, which are with respect to the main river of second-order influx. In the latter, third-order tributaries and so on, for example, in relation to p. Volga r. Kama is the influx of first order, and the p. Vyatka - the influx of second order.
Under the term, the main river in practice often understands any considered river, having an extensive network of tributaries.
The river system can be visually imagined as a hydrographic scheme (Fig. 8.2, a), on which all rivers are depicted by straight lines in the same scale. In each river, water flows from a certain territory called the river b and s s e n o m or o d o s b o r about m.
| but |  |
| B. |  |
Fig.8.2. River system:
a - hydrographic scheme of the river; B - River pool
The communal basin of the main river (Fig. 8.2, b) consists of private pools of all its tributaries and territories from which water flows into the main river (intermediate territory).
Pools of river systems and individual rivers are limited by closed lines, called in a d o r a d e l and m and. Watersheds are held according to the most sublime points of the area between adjacent rivers. Their position is installed on topographic maps of a large scale (1: 100000 or 1: 50000). If there is no horizontal on the map, the waterproof line is carried out approximately in the middle of the territories between adjacent rivers.
The main characteristic of the river basin is the value of its area, which is measured according to the card using a plan meter by 2-3 times the watershed stroke until the coincidences are obtained.
The area of \u200b\u200bthe catchment basin F (km 2) is within a certain dependence on the length L (km) of the main watercourse
F \u003d. to L m. (8.9)
According to statistical studies of the authors of this manual for the Basins of the Rivers of the Belarusian, the indicator of the degree M \u003d 1.68, and the coefficient to It changes in the range of 0.32-1.34, with its average value of it equal to 0.64, small values \u200b\u200bcorrespond to the pools with flat relief (Pripati's tributaries, Dnipro with L\u003e 150 km, Nemman), large - pools with a chronovoy-hilly terrain (Western Dvina, Pripal Functions for L< 30 км).
No wonder they say that it is necessary to look like water flows forever. I always liked to watch the strength of the rivers. They are like alive. True, rivers have a place of birth, long life, and in the end finish their journey, falling into the world ocean. Why there, they have their own way of life and their own river system!
How the river system is formed
The river begins his life from a small stream. Then the river grows and as the river begins to absorb other streams to absorb the river. The reservoir expands, and other rivers falling into it, and the former little stream becomes the main river. This is how the river system is formed.
Rivers and their tributaries are often formed intricate drawings. The hydrographic network is clearly seen from the air or on the map. Hydrographic networks have different configurations. The formation of the drawing of the river system affects:
- bias of terrain;
- climate;
- hardness of surface breeds, their looseness;
- geological history of the area.
Peculiar drawings of the river system
There are quite a few configurations of river systems. The most famous of them:
- radial;
- branched;
- rectangular.
I think that any geography amateur at least once was seen a branched network. It is she reminds all the tree. The branched system is spread on the area with a homogeneous rock (mainly clay).
The rectangular system is found in areas with solid rocks, hilly ridges and valleys. Such a system resembles a lattice.
The radial network visually looks like a wheel needles - rivers spread in different directions from the center. Another name of the network is centrifugal. You can observe a radial pattern in the area of \u200b\u200bthe mountains of a conical or dome-shaped form.
An important element of the water cycle
The water cycle begins with evaporation of water in the ocean and the formation of clouds. Clouds return the resulting moisture to the ground in the form of precipitation, a significant part of which falls into the ocean. The rest fall on land. The sediments can be drained from elevations, form sources, but they will return to the ocean over time through the river system.
The surface of the sushi on our planet is not really so dry. It is covered with a sufficiently dense network of water arteries - large and small rivers that provide a balance of moisture and feed on their waters of the forest, meadow and fields.
Depending on the climate and terrain, some regions are richer rivers, others are less. Do not be rivers, life on land is hardly so abundant and varied.
What is the river in geography?
The river is called a constant stream of large water water, the current one and the same natural channel, which is fed by drains of atmospheric precipitation and underground water sources. Sometimes the river can change the channel due to seasonal fluctuations in the water level or even rear during the summer drought, recovering again after the start of the rain period.
The beginning of the river is called, and the place where it flows into the sea or another river is a mouth. The height difference above sea level between the source and the mouth is customary to call the value of the river drop. For the mountain rivers, the fall can reach several kilometers even with a small length. By dividing the fall of the river or one of its sections for the length, you can get a slope value that is expressed as a percentage. 
The water level in the river is supported by the waterboat system - superficial and underground. In the underground catchment, underground sources are included, whose water fueled the river. Surface catchment form its tributaries - rivers and streams that fall into it throughout the river.
Atmospheric precipitates, falling on the surface of the sushi, flow from the hills in the lowest rashes of sushi, forming streams and small rivers in them. Those, in turn, flow even lower, gathering in the river bed.
What is a river system?
Any sufficiently large river has its own river system - the same name is the entire network of surface catchment, which includes not only the river itself, but all its tributaries, tributaries of the tributaries, etc. River systems are called the name of the largest, main river. Its tributaries are considered to be the tributaries of the first order, the tributaries of the tributaries - second-order tributaries, etc.
The main river must not fall into another river, but in a large water-sea or ocean, lake, swamp. Sometimes the main river water goes to the sand of the desert and accumulate in underground aquifers. Any river flowing into another river is secondary. 
Some rivers in their current share the channel into two parts, thus connecting two river systems. Such a phenomenon is called bifurcation. So, the Pijma River is divided into two sleeves, one of which flows into the Pechora and is called Pechora Pijm, and the other is in the mesten and is called the Mezen Pijm. Some bifurcation rivers are only in the period of flood - spring melting of snow.
Hydrographic system
The surface of the sushi occupied by one river system is called it. The borders of the pool often pass along the elevations or even mountain ranges and are called water-seed lines or simply watersheds.
All watercourses, both permanent and temporary, are connected into a row network, or a river system. Together with lakes, swamps, reservoirs and other reservoirs, it forms the hydrographic system of terrain on its territory.
Classification of rivers
In hydrology, it is customary to distinguish between mountain rivers flowing along a bed with a large bias, and plain rivers with a slow flow and a channel characterized by the presence of a large number of twists (meanders) with blurry and breed nans. By the size of the river are divided into:
- Large rivers - they include plain rivers with a pool of more than 50 thousand square meters. km or mountainous - with a catchment of more than 30 thousand square meters. km; 
- Middle rivers are predominantly plain, with a pool belonging to one hydrographic zone and having an area of \u200b\u200b2 to 50 thousand square meters. km;
- Small rivers whose pools are not exceeding 2 thousand square meters. km and belong to one hydrographic zone.
One river system, as a rule, consists of a large river, several medium and many small rivers.
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