Geomorphic impact of a beaver dam outbreak flood, Gatineau Hills, Quebec: a pictorial record

Abstract

On July 20th, 2007 Ottawa received 67.8 mm of rainfall, a near record amount for that date. Thirty kilometres to the north in the forested upland of the Canadian Shield, the intense rainfall caused two beaver dams to fail sequentially. The flow descended about 90 m in elevation along a < 1 km long, steep, rock-floored stream before it expanded across a bouldery, vegetated, and inhabited subaerial fan to discharge into Lake McArthur. The flood eroded soil, sediment, and vegetation along the length of the stream. Flow across the subaerial fan inundated one chalet to a depth of 1 m and rafted a smaller building several metres. Expansion bars were deposited at various flow expansion points; however, the most noteworthy deposit is a gravel delta that extends 10 m into the lake. Using a dam break model the peak discharge is estimated at about 70 m3 s-1 which is likely two orders of magnitude greater than average summer discharges. Given the coarseness of the bed material and the small discharge typical of the stream, it is likely that most bed material in the past has been mobilized during similar dam break floods. The study highlights the hazard posed by beaver dams to human infrastructure in steep terrain, and demonstrates that beaver dam outbreak floods can be the predominant agent of geomorphic change in small fluvial systems.

Wood and sediment storage and dynamics in river corridors

“…The effects of beaver dams on in‐channel entrainment thresholds and transport rates are similar to those described for other forms of LW. Beaver dams decrease sediment entrainment and transport within the backwater zone upstream from the dam (Butler and Malanson, ; John and Klein, ; De Visscher et al , ), enhance bed scour where flow plunges over a dam (Pollock et al , ), enhance spatial variation in average bed grain size (Curran and Cannatelli, ), and can create substantial pulses of sediment transport when a dam fails (Butler and Malanson, ; Russell et al , ).…”