The drainage basin system

A drainage basin is an area of land drained by a river and its tributaries. Around it lies the watershed, usually a ridge of higher land that separates neighbouring basins.

Because water and energy can cross its boundary, a drainage basin is an open system. Water arrives as precipitation, passes between stores and eventually leaves through river or atmospheric processes.

Inputs

Precipitation transfers water from the atmosphere to the basin. This can fall as rain, snow, sleet or hail.

The impact depends on its type and intensity. Intense rainfall may exceed the soil's infiltration capacity, producing rapid overland flow. Snow, however, can stay in cryospheric storage until it melts, delaying the input to the river.

Stores

Several parts of the basin can hold water temporarily:

• Vegetation storage: water intercepted by leaves, branches and stems.
• Soil-moisture storage: water held in spaces between soil particles.
• Aquifer storage: groundwater stored in permeable rock.
• Cryospheric storage: water stored as snow and ice.

Each store changes in size over time. Rainfall may replenish soil storage, for example, while evaporation, plant uptake and drainage into rock deplete it.

Flows

Infiltration is water moving from the ground surface into the soil. Rates are generally higher when soil is dry, permeable and well vegetated. They are lower when it is saturated, compacted or frozen.

Throughflow is the lateral movement of water through the soil towards a river channel. Since it is slower than most surface movement, it delays the river's response to precipitation.

Overland flow occurs when water moves across the ground surface towards a channel. This happens if rainfall exceeds infiltration capacity or saturated soil cannot hold any more water.

Base flow is the slow transfer of groundwater into a river. During periods with little or no precipitation, it maintains river flow.

Outputs

Evaporation changes liquid water on surfaces such as soil, vegetation and open water into water vapour, transferring it to the atmosphere. Transpiration releases water vapour from plants after their roots have taken up water. These atmospheric outputs are often grouped together as evapotranspiration.

Water can also leave the drainage basin as river discharge at its outlet. The amount available for channel flow depends on the balance between inputs, changes in storage and outputs.

Key terms: drainage basin, watershed, open system, precipitation, interception, vegetation storage, soil-moisture storage, aquifer, cryosphere, infiltration, throughflow, overland flow, base flow, evaporation, transpiration.

Discharge and stream flow

River discharge is the volume of water that passes a point in a river per unit of time. It is normally measured in cubic metres per second (m³/s) and calculated using the channel's cross-sectional area and its mean flow velocity:

Discharge = cross-sectional area × mean velocity

To estimate cross-sectional area, multiply the mean depth by the channel width. Discharge rises when the channel contains more water, when velocity increases, or when both happen together.

Stream velocity varies across a channel. Friction slows the water near the bed and banks, so the fastest flow is usually farther from these surfaces. Bends and obstacles can disrupt this pattern, as can irregular channel shapes.

Channel characteristics

Several channel characteristics affect the relationship between discharge and stream flow:

• Width and depth: larger channels can carry a greater volume of water.
• Channel shape: compared with a broad, shallow channel of the same cross-sectional area, a compact channel exposes less water to friction.
• Bed and bank roughness: large sediment and vegetation create turbulence. Irregular banks do the same, increasing friction and reducing velocity.
• Channel gradient: a steeper gradient generally increases the downstream component of gravity, which can increase velocity.

When discharge rises, water depth and cross-sectional area normally increase. Velocity may rise as well, since a smaller proportion of the water is in direct contact with the bed and banks.

Hydraulic radius

Hydraulic radius is the ratio between a channel's cross-sectional area and the length of its wetted perimeter:

Hydraulic radius = cross-sectional area ÷ wetted perimeter

The wetted perimeter is the length of the bed and banks that touches the water. With a high hydraulic radius, a relatively small proportion of the water touches the channel boundary. This reduces friction and generally makes flow more efficient. A low hydraulic radius shows proportionally greater contact with the boundary, so there is more friction.

A deep, relatively narrow channel usually has a higher hydraulic radius than a wide, shallow channel with the same cross-sectional area. However, hydraulic radius must be considered alongside gradient and roughness. An efficient channel may still flow slowly if it has a gentle gradient or a very rough bed.

Key terms: river discharge, stream flow, velocity, cross-sectional area, channel width, channel depth, gradient, roughness, wetted perimeter, hydraulic radius, friction.

River erosion

Erosion is the wearing away and removal of material by a river. Four processes operate within river channels:

• Hydraulic action: moving water forces material from the bed and banks; air compressed in cracks can help break it apart.
• Abrasion: sediment transported by the river scrapes and strikes the bed and banks.
• Attrition: transported particles collide, becoming smaller, smoother and more rounded.
• Solution: soluble minerals dissolve in the river water.

Which process dominates depends on velocity, turbulence, sediment load and the resistance of the bed and bank material. High-energy flow erodes more effectively because it applies greater force and can move larger particles.

Transportation

Transportation is the downstream movement of the river's load. A river carries material in four main ways:

A river's competence refers to the largest particle it can transport. Its capacity is the total quantity of load it can carry. Both generally rise as discharge and velocity increase.

Deposition

Deposition is the laying down of sediment carried by a river. It happens when the river loses energy, reducing its competence or capacity.

Velocity may decrease when the gradient becomes gentler, discharge falls, the channel grows wider or shallower, friction increases, or the river enters still water. Larger, heavier particles are normally deposited before finer material.

Spatial variation

River processes change along the channel. In some reaches, steep gradients, coarse bed material and turbulent flow may make erosion important. Farther downstream, greater discharge can raise transport capacity. Gentler gradients and entry into low-energy environments, however, can encourage deposition.

Local differences develop because of channel characteristics. Pools, riffles, bends, changes in width and depth, roughness and obstacles redistribute velocity. As a result, one side of a single reach may experience erosion while deposition occurs on the other.

Temporal variation and seasonality

River energy also changes through time. During a wet season or snowmelt period, higher discharge and velocity can increase erosion, competence and transport capacity. Flood flows may entrain sediment that stays immobile under ordinary conditions.

In a dry season, lower discharge usually reduces competence and capacity, which encourages deposition. The timing and importance of these changes depend on the basin's seasonal pattern of precipitation, temperature and snow or ice melt.

Key terms: erosion, hydraulic action, abrasion, attrition, solution, transportation, traction, saltation, suspension, load, competence, capacity, deposition, channel characteristics, seasonality.

Waterfalls

A waterfall is a near-vertical drop in a river's long profile. Waterfalls commonly form where resistant rock lies above less resistant rock.

Hydraulic action and abrasion erode the less resistant rock more quickly. This creates a step and undercuts the harder cap rock. At the base, turbulent water and transported sediment carve out a plunge pool. Eventually, the unsupported resistant rock collapses. As this process repeats, the waterfall retreats upstream and leaves behind a steep-sided gorge.

Meanders

A meander is a winding bend in a river channel. Small irregularities shift the fastest current from one side to the other, creating different flow conditions across each bend.

Water is deeper and faster on the outside bend, so hydraulic action and abrasion form a steep river cliff. On the inside bend, the water is shallower and has less energy. Sediment is deposited there, forming a slip-off slope. Helicoidal flow carries water and sediment across the channel and strengthens the bend. Continued erosion of the outer bank and deposition on the inner bank cause the meander to migrate laterally and downstream.

Floodplains

A floodplain is the broad, relatively flat valley floor beside a river that is periodically covered by floodwater. It forms through repeated overbank deposition and the lateral migration of meanders.

As bends change position, lateral erosion widens the valley floor. When discharge exceeds the channel's capacity, water spreads across the valley. It becomes shallower and meets greater surface friction, so its velocity falls. Fine alluvium is then deposited in layers. Over time, repeated floods build a wide, fertile floodplain.

Levees

Levees are raised banks of river sediment beside a channel. Natural levees develop during repeated floods.

When floodwater leaves the channel, friction increases and velocity drops sharply. Coarser material is deposited first, close to the banks. Finer sediment travels farther across the floodplain. Repeated flooding builds ridges that are highest beside the channel and become lower farther away.

Deltas

A delta is an accumulation of sediment at a river mouth where the river enters a lake or sea. As the river flows into slower or standing water, it loses velocity, reducing its competence and capacity.

Coarser sediment is deposited first. Finer material travels farther from the river mouth. As the sediment builds up, it obstructs the channel and causes it to divide into distributaries. Continued deposition extends the delta outward.

Deltas grow most readily where sediment supply is high and deposition occurs faster than waves, currents or other water movement can remove material. Shallow water also makes it easier for deposited sediment to build above the surface.

Key terms: waterfall, resistant rock, undercutting, plunge pool, gorge, meander, river cliff, slip-off slope, helicoidal flow, floodplain, alluvium, levee, delta, distributary, erosion, transportation, deposition.