How and why does the water in a river fluctuate?

The discharge of a river shows much variation during a year and in the short term. It can fluctuate a lot in a matter of hours in response to periods of rain.
Hydrological Cycle diagram
A video on the hydrological cycle –

River discharge
River discharge is defined as the volume of water passing a measuring point or gauging station in a river in a given time. It is measured in cubic metres per second (cumecs).
The storm hydrograph
The storm hydrograph (shown to the left) shows variations in a river’s discharge over a short period of time, usually during a rainstorm. The starting and finishing level show the base flow of the river. As storm water enters the drainage basin the discharge rises, shown by the rising limb, to reach the peak discharge, which indicates the highest flow in the channel. The receding limb shows the fall in the discharge back to the base level. The time delay between maximum rainfall amount and peak discharge is the lag time.
Changes in discharge
Hydrographs are graphs which show river discharge over a given period of time and show the response of a drainage basin and its river to a
period of rainfall.
A storm hydrograph shows how a river’s discharge responds following a period of heavy rainfall. On a hydrograph, the flood is shown as a peak above the base (normal) flow of the river. Analysis of hydrographs can help hydrologists to predict the likelihood of flooding in a drainage basin. The response of a river to a rainfall event can be measured in terms of the lag time – the time between peak rainfall and peak discharge.
Rivers with a short lag time respond rapidly to rainfall events and are therefore more prone to flooding than rivers with a longer lag time
River discharge does not respond immediately to rainfall inputs as only a little of the rainfall will fall directly into the channel. The river will start to
respond initially through inputs from surface runoff (the fastest flow of water) and its discharge will later be supplemented through inputs from throughflow and
groundwater flow
The fastest route to the river is via overland flow. If most of the water in a drainage basin travels in this way, a river will respond quickly to heavy rainfall and the hydrograph shape will be ‘peaky’ (graph A) with steep rising and recessional limbs. The lag time will be short and there will be a greater risk of flooding. Where more water is able to pass into the soil and travel to the river via throughflow / groundwater flow, there will be a slower rise in discharge and the river will respond slower (graph B). The lag time will be longer and the risk of flooding will be much lower
Factors affecting a river’s discharge
Rock and soil type
· Permeable rocks ad soils (such as sandy soils) absorb water easily, so surface run-off is rare
· Impermeable rock and soils (such as clay soils) are more closely packed. Rainwater can’t infiltrate, so water reaches the river more quickly
· Pervious rocks (like limestone) allow water to pass through joints, and porous rocks (like chalk) have spaces between the rock particles
Land use
· In urban areas, surfaces like roads are impermeable – water can’t soak into the ground. Instead, it runs into drains, gathers speed and joins rainwater from
other drains – eventually spilling into the river
· In rural areas, ploughing up and down (instead of across) hillsides creates channels which allow rainwater to reach rivers faster increasing discharge
· Deforestation means less interception, so rain reaches the ground faster. The ground is likely to become saturated and surface run-off will increase
· The amount and type of rainfall will affect a river’s discharge
· Antecedent rainfall is rain that has already happened. It can mean that the ground has become saturated. Further rain will then flow as surface run-off towards the river
· Heavy continual rain, or melting snow, means more water flowing into the river
· Steep slopes mean that rainwater is likely to run straight over the surface before it can infiltrate. On more gentle slopes infiltration is more likely.
Weather conditions
· Hot dry weather can bake the soil, so that when it rains the water can’t soak in. Instead, it will run off the surface, straight into the river.
· High temperatures increase evaporation rates from water surfaces, and transpiration from plants – reducing discharge
· Long periods of extreme cold weather can lead to frozen ground, so that water can’t soak in