How a Watershed Dam WorksFlood Control and MoreTheWatershed Program (Watershed Protection andFlood Prevention Act, Public Law 83-566) andFlood Control Act of 1944, Public Law 78-534)helps communities and rural areas reduce floodingand sedimentation, provides waters supplies andrecreational areas and creates thousands of acresof wildlife habitat.There are watershed projects inall 50 states and the Caribbean. Since 1948, 2,000watershed projects, covering160 million acres, havebeen organized by local project sponsors withassistance from the USDA Natural ResourcesConservation Service.

Eleven thousand watershed dams have been builtin 47 states. Watershed projects make up a $15billion national infrastructure that provides $2billion in annual benefits to over 47 million people.

Watershed lakes average from about 5 to 25 acres in surfacearea. A few are larger, up to several hundred acres and someare designed as dry structures with no permanent water.

How Watershed Dams Reduce FloodingThe concept of watershed projects is simple. Aseries of watershed dams are built across smalltributaries to larger streams. The dams temporarilystore flood water after rain storms and slowlyrelease it over a period of several days through apipe in the dam. This reduces the amount of waterthat reaches the main water course after a rain,reducing flooding.

Land treatment programs in the watershed helpcontrol erosion, which reduces the sediment thatflows into the streams and lakes.

Watershed projectsusually consist ofearthen damsconstructed ontributaries to a river.The number of damsin a watershed variesdepending on thesize of thewatershed. Someprojects do not havedams, but use otherconservationmethods to meet theneeds in a watershedsuch as erosioncontrol, animal wastemanagement, and water quality practices.

A concrete inlet tower (might be metal on older dams)connected to a pipe extending through the dam serves as aprincipal spillway for dams, controlling the water level. Aslide gate at the bottom of the inlet can be opened to lowerthe water level for maintenance and repair.

Water is released through a pipe in the dam for several daysafter a heavy rainstorm.

Cross Section of a Watershed Dam

This cross section of a dam shows the concrete inletin front of the dam connected to the principalspillway pipe (extending through the dam). Whenwater reaches the opening at the top of the inlet itspills over and goes through the pipe controllingthe level of water in the lake. Some inlets also haveopenings in the side of the tower.

A slide gate, located at the bottom of the inlet tower,can be open to lower the water level formaintenance and repairs.

The diagram also shows the auxiliary spillway level.Water will flow through the earthen spillway at theend of the dam to safely convey large flows to avoidwater going over the top of the dam, which coulddam failure.

The sediment storage area on the cross section iswhat makes up the permanent pool of water. Overthe life time of the dam this area will usually fillwith sediment (usually 50 to 100 years).

The National Watershed Coalition is a nonprofit organization composed of national, regional, state and localassociations, organizations and individuals who advocate using watersheds as the planning and implementation unitwhen dealing with natural resource problems. Visit our website at: www.watershedcoalition.org

National Watershed Coalition

August 2006

The red line on thisphoto indicates the extent that watercan back up in the flood pool upstream before it startsflowing through the earthen spillway. This is whereflood water is stored while it is slowly released throughthe principal spillway pipe.

It is important to keep the inlet tower clear of debrisand to keep the earthen spillway clear of any structuressuch as fences and buildings. Disrupting the flow ofthe spillway could result in dam failure.

When there is more water than the reservoircan store and the tower inlet can release,water will flowthrough the earthen spillway.

Flood Storage

inlet