Many rivers are fed by springs, which occur at points where groundwater reaches the surface. Springs can occur in different geological settings, forming valley springs, stratum springs or solution channel springs.
The water in a river originates from overland flow, from interflow and from baseflow. Baseflow forms a higher proportion of river water in summer than in winter, and in rivers flowing over good aquifers.
River discharge at a particular point is usually determined by measuring the stage, which is the water level in the river, and then reading off a value for the discharge from the rating curve — a plot of measured discharge for various stages. A river discharge hydrograph is a record of the discharge over a period of time. The shape of a short-period hydrograph (the record for a few days) depends on the size, shape, geology, vegetation and land use of the river catchment. The shape of the long-period hydrograph (e.g. for a year) depends primarily on the type of climate in the river catchment.
Reservoirs increase the amount of water stored on the land surface. They can be used as direct supply reservoirs or for river regulation. Reservoirs may also be built solely or partly for other purposes, such as the generation of hydroelectricity or for flood prevention.
The criteria for selecting sites for water-supply reservoirs are: a good supply of high-quality water, minimum ecological and environmental disturbance, a high elevation, a watertight reservoir area, no geological hazards and a suitable dam site. The most suitable reservoir sites are narrow, deep valleys, but reservoirs often have to be built in wider valleys or in flatter lowland areas.
There are two types of dams, gravity dams and wall dams. The gravity dam depends on its own weight to maintain stability, whereas the wall dam is a rigid structure that transfers the pressure of the water to the floor and sides of a valley.
The environmental effects of constructing a reservoir include the loss of a large area of land, ecological changes, dam failure, sediment filling, sediment loss to agriculture, soil salinization and induced earthquakes.
Reservoir projects involving big dams are becoming increasingly subject to scrutiny, particularly on the grounds of sustainable development.