ASDSO Dam Safety Toolbox

Unusual or Changing Conditions in a Watershed

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Unusual or changing conditions in the watershed may result in larger than expected runoff events that can surprise owners and operators. These conditions may include rain on snow events, frozen or saturated ground, burned watersheds, and more. Urban and agricultural development within a watershed can also lead to measurable effects in hydrologic conditions such as the runoff ratio and the time of concentration. While it is essential that owners, engineers, and stakeholders are acutely aware of changes occurring at the dam, it is also important that they are aware of changing conditions throughout the watershed and attempt to accurately predict how those changing conditions may affect the operation of the dam.

“Streamflow at any particular location can be greatly affected by hydraulic structures located upstream. It is important, therefore, that essential data be obtained on all significant hydraulic structures located in and upstream from a study area. For diversion structures, detailed data are required on the size of the diversion dam, capacity of the diversion canal, and the probable size of flood required to washout the diversion dam. In the case of storage reservoirs, detailed data on the relation of storage capacity to elevation, location, and size of outlets and spillways, types, sizes, and operation of control gates, and sizes of power plant and penstocks should be known. Bridges can produce backwater effects which will cause upstream flooding, This flooding may be produced by the approach roads, constriction of the channel and floodplain, pier shapes, the angle between the piers and the streamflow, or the pier length-width ratios”.[1]

In some cases, canals may traverse a watershed and import additional flow to the reservoir from outside of its natural drainage area during hydrologic events. Sometimes, peak flows in canals may eclipse runoff from the basin and should be used as the basis for spillway capacity if it is plausible that the canal banks could fail during a hydrologic event.


Learn more about the impacts of unusual or changing conditions in a watershed at

Best Practices Resources

Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE


Revision ID: 7475
Revision Date: 07/28/2023