Factors Affecting River Landscapes and Hydrology

A river is connected to its whole drainage basin

A river landscape is shaped by more than water flowing in a channel. Rainfall supplies water, the underlying rocks influence erosion and drainage, and material moves down valley sides into the river. These processes connect the surrounding drainage basin—the land drained by a river and its tributaries—to the channel.

Two useful measures are discharge, the volume of water passing a point each second, and sediment load, the solid material carried by the river. Physical factors affect both how much sediment becomes available and whether the river has enough energy to transport it. Supplying more sediment does not necessarily mean it will all be carried away: some may be deposited if the flow cannot transport it.

Climate controls water supply and weathering

Climate describes the usual weather conditions of an area over time. Frequent rainfall can maintain river discharge, while intense storms can produce a sudden increase. Greater discharge generally gives a river more energy for erosion and sediment transport. Storm flows can therefore erode channels and move material that would remain stationary during lower flows.

In drier conditions, less water reaches the river. High temperatures also increase evaporation, which can reduce discharge. With less energy available for transport, the river may deposit some of its load.

Climate also affects sediment supply through weathering, the breakdown of rock in place. Where temperatures repeatedly rise above and fall below freezing, water in cracks freezes and expands, gradually breaking rock apart. This freeze–thaw weathering produces fragments that can later move down valley sides and enter the channel.

Snow temporarily stores precipitation rather than immediately supplying a river. Discharge may rise later when the snow melts; rapid melting can deliver a large amount of water over a short period.

Geology affects both erosion and drainage

Geology includes the types and structure of the rocks beneath a landscape. Two properties matter here: resistance to erosion and permeability.

Resistant rocks, such as granite, erode relatively slowly. Less resistant rocks, such as clay and shale, are more easily worn away, helping valleys widen and supplying sediment. Contrasts in resistance can also help create waterfalls where a river crosses from harder to softer rock. Rock type influences landscape development, but does not determine every landform by itself.

Permeability describes how easily water passes through a material. Permeable rock, such as chalk, allows water to enter the ground and move towards the river more slowly. Relatively impermeable rock, such as slate, restricts this movement, encouraging more water to travel over the surface.

These are different properties: knowing that a rock resists erosion does not, by itself, tell you how easily water passes through it.

Valley slopes supply sediment

Weathering prepares loose material on valley sides. Mass movement then transfers rock and soil downhill under gravity, changing the valley profile and potentially adding material to the river.

A slide moves a mass of material down a slope, often along a relatively flat surface. A slump is a rotational movement along a curved surface, common in weak material such as clay. Prolonged or heavy rainfall can make slopes saturated and heavier, increasing instability. Soil creep is much slower, gradually moving soil downhill.

Steep slopes encourage faster surface runoff, which can wash loose material towards the channel. Steep channel gradients also favour energetic erosion and transport. Where flow slows and loses transport capacity, deposition becomes more likely. The landscape therefore develops through the interaction of valley-side processes and processes within the river itself.

Reading a storm hydrograph

A storm hydrograph shows how river discharge changes during and after a rainfall event. Rainfall is commonly shown as bars and discharge as a curve, both against time. Their vertical scales are different: rainfall may be measured in millimetres, while discharge is measured in cubic metres per second.

The rising limb is the part of the curve where discharge increases as water reaches the channel. Peak discharge is the highest discharge during the event. The falling limb shows discharge decreasing as the basin drains after the storm.

Lag time is the interval between peak rainfall and peak discharge. The discharge peak usually comes later because water takes time to travel through the drainage basin.

A steep rising limb, short lag time and high peak indicate a rapid, concentrated response. A gentler rise, longer lag time and lower peak indicate that water arrives more slowly and is spread over a longer period. A high peak may increase flood risk, but flooding also depends on whether discharge exceeds the channel’s capacity.

Why some basins respond faster than others

Geology and soil type: permeable ground encourages infiltration and slower movement through the soil and rock. This tends to lengthen lag time and reduce peak discharge. Poorly permeable ground, including clay-rich soil, restricts infiltration. More water then runs over the surface, reaching the river quickly and producing a shorter lag time and higher peak.

Slope: on steep valley sides, runoff travels downhill quickly. Water reaches the channel sooner, tending to produce a steeper rising limb and shorter lag time. Gentler slopes slow its movement.

Drainage basin shape: in a roughly circular basin, water from different parts of the basin tends to reach the outlet at similar times. These flows combine to produce a relatively high peak and short lag time. In a long, narrow basin, arrival times are more spread out, tending to produce a lower peak and longer lag time. This comparison is most useful when basin area and other conditions are similar.

Antecedent conditions: soil already saturated by earlier rain has little capacity to take in additional water. More of the next storm becomes surface runoff, shortening lag time and increasing peak discharge. Drier soil usually has more available storage, although soil permeability still matters.

Rainfall characteristics interact with these basin properties. Intense rainfall may arrive faster than the ground can absorb it, while prolonged rain can fill soil storage. For example, a steep basin with poorly permeable ground that is already saturated would tend to respond rapidly to a storm. Its quick runoff could also increase erosion and move sediment supplied by unstable valley sides.

Landscape and sediment controls

• Climate: rainfall increases discharge and transport energy; dry, hot conditions can reduce flow. Freeze–thaw supplies rock fragments; snowmelt can increase discharge.
• Geology: less resistant rocks erode more readily. Permeability controls how water enters and moves through the ground.