Three linked water bodies
Fresh water on land is kept in three main places. Each one is a store.
- River: water flowing in a channel from high land to low land. It is a fast-moving store.
- Lake: water standing still in a hollow of the land. It is a slower store.
- Groundwater: water that has soaked into soil and rock under our feet. It is the slowest store but the biggest one. The top of the soaked layer is the water table.
There is a fourth store in cold places: snow and glaciers. They hold water as ice and release it when they melt.
The sources are simple. Rain, snowmelt and glacier melt add water. Evaporation and flow to the sea take it away.
Recharge: who feeds whom?
Recharge means one water body gives water to another. The rule is easy: water moves from the higher level to the lower level.
Wet season: the river feeds the ground
After heavy rain the river level goes up fast. It becomes higher than the water table beside it. River water seeps through the bed and banks into the soil. The water table rises. This is river recharge of groundwater. Wells near the river fill up.
Dry season: the ground feeds the river
When rain stops, the river falls quickly. The water table falls slowly, so it stays higher than the river. Groundwater seeps out into the river bed. This slow supply is called baseflow. It is why many rivers keep flowing for weeks without rain.
Lakes
A lake works like a big tank. In floods it fills and the river peak is smaller. In dry months it releases water to the river and to the ground below. A lake can also take water from the ground when the water table is higher than the lake.
Snow and glacier melt
In high mountains and dry regions, summer melt gives rivers their main water. The river then recharges the groundwater of the plains below. The three bodies are one connected system, so a change in one changes the others.
What happens when people interfere?
Because the stores are linked, our actions travel through the system.
- Too many wells pull the water table down. A river that was fed by the ground (a gaining river) can turn into one that only loses water to the ground, and may dry up.
- Dams store floods and give a steady flow, but the river below may recharge the ground less.
- Cutting forests and covering land with concrete lets rain run off at once. Less water soaks in, so groundwater is not refilled.
- Pollution in a river or lake can seep into groundwater and spoil wells.
Good management means taking from the ground no more than rain and rivers can put back, protecting wetlands and forests, and planning water use for the whole river basin together.
Try it
- Fill a clear box with sand. Dig a long groove and pour water into it as a "river". Watch the sand beside it turn dark as the water table rises.
- Now remove the water from the groove. Does the dark sand keep some water for a while? That is groundwater feeding a river.
- In the 3D free-play step, slide to "wet". Which way do the arrows point? Slide to "dry" and check again. At which slider position do the arrows disappear?
Key formulas and definitions
- Water always moves from the higher level to the lower level
- River higher than water table: river recharges groundwater (losing river)
- Water table higher than river: groundwater feeds the river (gaining river, baseflow)
- Water balance of a basin: rain = river flow + evaporation ± change in storage
- Residence time = volume in store ÷ rate of flow out
Worked examples
1. A well beside a river has its water 8 m below ground in May and 3 m below ground in September. What happened to the water table, and why?
The water table rose by 8 − 3 = 5 m. The monsoon rain filled the river and the river water also soaked into the ground, so the river recharged the groundwater.
2. Rain stopped 40 days ago but a river is still flowing. Where is its water coming from?
From groundwater. The water table is higher than the river level, so water seeps into the river bed. This slow supply is baseflow.
3. A lake of area 4 km² falls by 2 m in a dry spell with no rain. How much water did it lose? (1 km² = 1,000,000 m²)
Volume = area × drop = 4,000,000 m² × 2 m = 8,000,000 m³. That is 8 million cubic metres, or 8,000 million litres.
Common mistakes
- Thinking that a river always gives water to the ground. In the dry season it is often the ground that gives water to the river.
- Thinking groundwater is an underground river. It is water held in tiny spaces of soil and rock and moves very slowly.
- Forgetting that the water table moves up and down with the seasons.
- Studying river, lake and groundwater as separate things. They exchange water all the time.