FLOOD WAVE ATTENUATION BY A WETLAND FOLLOWING A BEAVER DAM FAILURE ONA 2ND-ORDER BOREAL STREAM

On June 8 1993, a beaver dam on Rocky Creek, a small stream in central Alberta, failed and released about 7500 m(3) of water. The estimated peak of the resulting flood wave was 15 m(3) s(-1), which is 3.5 times the maximum discharge recorded for the creek over 23 years. The flood wave destroyed five hydrometric stations, scoured some channel reache s, and deposited sediment in others. Large trees and debris from old b eaver dams were carried downstream and deposited in piles across the c hannel and adjacent banks. The flood wave peak was dampened to 6% of t he estimated upstream flood peak as it passed through a 90-ha wetland characterized by both organic and mineral soils, sedge meadow and will ow, and containing a small lake and several beaver ponds. Although the peak flow resulting from the dam outburst was extreme, the volume, mo st of which was probably detained in the wetland, was not. The volume amounted to only 7% of the combined basin runoff from precipitation an d the beaver dam outburst. This extreme event showed that beaver activ ity can greatly affect the hydrology of small streams and demonstrated the importance of taking wetland storage into account when conducting hydrologic studies on watersheds that contain sizeable areas of wetla nds.