RIVERS DRAINAGE BASINS

Important Definitions / Notes Tributary: a stream or river that joins a larger river. Confluence: the point at which two rivers meet. Precipitation: any water that falls from the atmosphere viz. rain snow, sleet or hail. Headwaters / Source: the furthest point along the course of a river from its estuary, mouth or confluence with another river i.e. where the river starts.

Confluence and tributary of Sligachan River, Isle of Skye. Pollution at the mouth of the Millers River where it enters the Connecticut River. Millers River is the tributary and its mouth is also a confluence.

Important Definitions / Notes Headwaters may be in a marsh or formed from glacial melt waters, in a lake or spring. Mouth: the part of the river that flows into a lake, reservoir, sea, ocean or estuary. Groundwater: water stored underground in permeable rock such chalk or sandstone.

The source of Nile from the underwater spring at the neck of Lake Victoria, Jinja.

The source of the Irrawaddy River – Lake Laka

The source of the Rhume River in Germany is a karst spring

Source of the River Ethiope, Nigeria – deepest inland waterway in Africa – is a spring at the base of a silk cotton tree

El Nevado Mismi: This ice mountain is on the continental divide, one side drains through the Amazon River to the Atlantic Ocean, and the other to the Pacific Ocean.

The source of the River Severn – the longest river in England - is a deep, blanket-peat bog in Wales.

Mouth of Cass River in Lake Tekapo Mouth of Hoh River

Features of a Drainage Basin

What is a drainage basin? 1st Definition 2nd definition An extant or area of land within which surface and groundwater from rain, melting snow or ice is transferred to a single point at lower elevation, usually the exit of the basin, where the waters join another water body, such as a river, lake, reservoir, estuary, wetland, sea or ocean. The area of land drained by a river system i.e. a river and its tributaries. It includes water found in the water table as well as surface runoff.

What is a drainage basin? Drainage basins drain into other drainage basins in a hierarchical pattern, with smaller sub-drainage basins  or sub-catchments combining into larger drainage basins. The drainage basin acts as a funnel, collecting all the water within the area covered by the basin and channeling it to a single point such as the ocean, a lake or a larger stream. Tributaries and the main stem river serve to drain the surrounding drainage basin of its surface water and groundwater by leading the water out into an ocean, sea, lake, estuary, reservoir or another river. Each drainage basin /sub-basin is separated topographically by a perimeter called a drainage divide.

An example of a drainage basin. All water collected within the area enclosed by the dashed line is channeled to a single point. The dashed line is the main water divide or drainage divide.

Main and sub-basins

What is a drainage basin? In a drainage basin the tributary stream (A) of a brook (B) that joins a small river (C) is the tributary of a larger river (A is tributary of B, which is larger than A. B is a tributary of C, which is larger than B and A) For water to flow from A to B, the drainage basin of A has to be at a higher elevation than B. For the same reason, B is at a higher elevation than C. A, B and C are thus a part of a series of successively smaller area but higher elevation drainage basins. Similarly, the Missouri River is part of its own drainage basin and that of the Mississippi River.

What is a drainage basin? Other terms that are used to describe drainage basins are: catchment catchment area catchment basin,  drainage area,  river basin water basin. In North America, the term watershed is commonly used to mean a drainage basin, though in other English-speaking countries, it is used only in its original sense i.e. to mean drainage divide

What is a drainage basin? Drainage basins are the main unit used in the study of rivers and the movement of water within the hydrological cycle. This is because the majority of water that discharges from the basin outlet originated as precipitation falling on the basin. Drainage basins are separated from each other by watersheds or drainage divides. These are the boundaries between drainage basins: all the precipitation on opposite sides of a drainage divide will flow into different drainage basins.

What is a drainage basin? Endorheic Basins: these are closed drainage basins. Areas of inland drainage that allow no outflow to external bodies of water like oceans or seas. Water only leaves by evaporation or seepage. Water will converge to a single point within the basin called a sink. A sink may be a permanent lake, seasonal lake, swamp or an area where water is lost underground. The bottom of such a basin is usually occupied by a salt lake or salt pan.

Endorheic basin showing waterflow input into Üüreg Lake, western Mongolia.

The Caspian Sea is the world’s largest lake The Caspian Sea is the world’s largest lake. A large part of Eastern Europe, drained by the Volga River, is part if its drainage basin.

Okovango Delta, Botswana Okovango Delta, Botswana. Dead Sea and Sea of Galilee, Jordan – Israel Border.

Watershed An area or ridge of land that separates waters flowing to different rivers, basins, or seas. A watershed is an imaginary line separating adjacent basins. Streams on either side of a watershed will flow in opposite directions. In hilly areas, the divide lies along ridges and may be in the form of a single line of hills or mountains known as a dividing range. In flat areas, the location of the divide is harder to discern.

Watershed Continental Divide: where waters on each side flow to different oceans e.g. the Congo- Nile Divide. Major Drainage Divide: waters on both sides do not meet but flow into the same ocean e.g. the Yellow and Yangtze River Divide Minor Drainage Divide: waters part but eventually rejoin e.g. the Mississippi and Missouri River Divide.

Diagram of a drainage basin showing the watersheds or divides. The position of a watershed is usually shown on maps/diagrams as a dotted/dashed line. In reality, the watershed will take the form of a ridge. Thus precipitation falling on such a ridge will flow (above or below ground) in opposite directions.

Global Hydrological Cycle This is the name given to the continuous movement of water between the atmosphere, lithosphere and biosphere. The total mass/volume of water remains constant over time. The partitioning of water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable and dependent on a wide range of climatic variables. Thus, it is a closed system, with no inputs and outputs or losses. Instead there are transfers between stores.

Global Hydrological Cycle Water moves from one reservoir to another e.g. river to ocean or ocean to atmosphere, by the physical processes of: Evaporation Condensation Precipitation Infiltration Runoff Sub-surface flow

Basin Hydrological Cycle In both hydrology and fluvial geomorphology, the drainage basin is the main unit of focus for study. Thus, when studying rivers, frequent mention is made of the basin’s hydrological cycle. When doing this, the drainage basin is said to be the unit of study as opposed to the global system. A drainage basin is a local open system, with inputs, outputs and processes/transfers.

Basin Hydrological Cycle At the local scale the cycle has a single major input – precipitation (PPT). There are two major losses or outputs – evapotranspiration (EVT) and runoff. Water is stored at a number of places within the system – vegetation, surface, soil moisture, groundwater and water channels.

The basin hydrological cycle is also an open system! Inputs Outputs Precipitation (including rain and snow) Solar energy for evaporation and transpiration. Evapotranspiration from vegetation Run-off into the sea Percolation of water to underlying rock strata into underground stores

The basin hydrological cycle is also an open system! Stores Transfers/Flows Surface storage such as puddles, rivers and lakes Glaciers Soil storage and ground water storage Water stored on vegetation (interception) following precipitation. Percolation Overland flow Infiltration Stemflow Throughflow

Human modification of the system Large-scale changes of channel flow Irrigation Drainage Abstraction of groundwater and surface water for domestic and industrial use