The flow of a river is quite irregular when considered over long time periods.
It is characterized by rises from rainfall and snowmelt followed by gradually receding flow.
The most elementary river and flood forecasts are concerned with predicting the time and height of stages caused by peak flows.
A recent trend has been toward more and more detailed forecasts with continuous flow forecasting required by more and more users.
Continuous flow/stage forecasts enable the user to make decisions as to actions they may wish to take with regard to high or low flows.

There are three major steps required to forecast the hydrograph that results from a rainstorm or period of heavy snowmelt.
(1) An accurate estimate of the volume of water that will run directly off the land surface into the stream must be made.
(2) The distribution of this volume of water, with time as it passes a forecast point, must be determined.
(3) The change in shape of the hydrograph as the volume of water moves to points further downstream must be calculated.

The quantity of runoff from a given storm is dependent on moisture conditions in the basin prior to the storm, characteristics of the storm itself and, to some extent, season of the year.
A common procedure used by the NWS for determining runoff is the Antecedent Precipitation Index (API) technique.
It is based on the assumption that soil moisture decreases logarithmically with time during periods of no precipitation.
Using API's and coaxial rainfall-runoff relationships, runoff can be calculated.

Soil moisture accounting models, such as the Sacramento Model, represent the upper layers of the soil mantle.
By constantly monitoring the movement of moisture in and out of the soil mantle, runoff is determined.
Unit hydrograph theory assumes the hydrographs for given locations reflect the characteristics (e.g., shape, size, slope) of that unique basin and that similar storms in the basin will produce similar hydrographs.
Multiplying runoff times the unit hydrograph determines the forecast hydrograph for the location in question.
Routing of the hydrograph downstream can then be accomplished by a number of well-documented techniques including dynamic routing.

A.G.Cudworth, Flood Hydrology Manual, WRTP,USGS,1989 - Dept. of Interior, pg65