River Regime

River discharge is a measurement of the amount (volume) of water passing through the river channel at any one time. From the river discharge, we can find out the rate of flow of a river. To calculate the river discharge, the following calculation is used:

This equation makes it clear that the components of river discharge are the volume of water (and other materials) passing through a river and the speed at which it travels. Like the river regime, river discharge is affected by the variability of the climate and the properties of the river.

Figure 1: Average discharge of the world's largest rivers, Ashersea/Wikimedia

By definition, there are multiple factors that can impact the river regime over time and space.

A drainage basin is an area drained by a river. They vary in many ways, including in size, shape, rock type, vegetation coverage and more.The shape of the drainage basin can significantly impact the river regime. For example, if the sides of the drainage basin are steep, then water can get to the river more quickly because of the faster flow over the land (overland flow).

If the shape of the drainage basin is more circular, then the time taken for water throughout the drainage basin is, on average, shorter. Again, this would influence the river discharge to be faster. This is because, in a more rectangular basin, some water from the furthest points would take a much greater amount of time to reach the river channel.

As the river regime refers to changes in the river discharge, the drainage basin properties only really contribute if they change. This can occur as a result of processes such as erosion, weathering and deposition. All of these processes can impact drainage basin properties by either facilitating or slowing the movement of water from around the drainage basin to the river. For example, erosion and weathering have the potential to alter the gradient of the sides of the drainage basin. If erosion or weathering happens to make the gradient steeper, it will impact the river regime.

Figure 2: The water budget of (parts of a) watershed. Source: USGS.

Land use & vegetation

Another characteristic of drainage basins that influences the river regime is land use and the presence or absence of vegetation. This is because of the relationship between these factors and the movement of water from around the drainage basin to the river itself. Let’s explore how this works:

The main property of the river that affects the river discharge and, by extension, the river regime is the cross-sectional area. The cross-sectional area of a river can be changed over time for many reasons. This can be through the erosion of the banks or river bed or through the deposition of rocks or other matter that can obstruct the flow of water. These changes can alter the river discharge over the course of a year by affecting the volume of water that can pass through the channel at any one time.

The balance between precipitation and evapotranspiration alters the river regime. Precipitation and evapotranspiration are key controls of river discharge because they directly impact the amount of water in the river system. Where precipitation adds water, and so has the potential to increase river discharge, evapotranspiration does the opposite and can decrease the river discharge. Precipitation and evapotranspiration are variable over the course of the year.

In the UK, typical patterns include higher levels of precipitation in the winter months and higher levels of evapotranspiration in the summer months. However, extreme weather events can cause heightened precipitation and/or evapotranspiration throughout the year. For example, a river's peak discharge often coincides with a storm event. This balance between the two processes significantly influences the river regime. This also relates to and affects the water balance because it alters the inputs and outputs of water in the system.

Another element of the climate that impacts the river regime is temperature. Temperature is a key determinant in the levels of evapotranspiration. Higher temperatures are associated with higher levels of evapotranspiration. This is because the associated increased amount of energy allows water to change its state from a liquid to a gas. As we have discussed already, evapotranspiration and its annual variability is a key factor affecting a river regime. Therefore, the temperature is also an indirect influence.

It is important to understand the river regime for many reasons. The main reason is that it can help with river management and ensure that the catchment area and its inhabitants are protected from damage. For instance, if the river regime suggests that severe flooding could happen in the winter months, strategies ought to be implemented to reduce the damage done.