Beaver dams, hydrological thresholds, and controlled floods as a management tool in a desert riverine ecosystem, Bill Williams River, Arizona

Abstract

Beaver convert lotic stream habitat to lentic through dam construction, and the process is reversed when a flood or other event causes dam failure. We investigated both processes on a regulated Sonoran Desert stream, using the criterion that average current velocity is <0Ð2 m s 1 in a lentic reach. We estimated temporal change in the lotic : lentic stream length ratio by relating beaver pond length (determined by the upstream lentic-lotic boundary position) to dam size, and coupling that to the dam-size frequency distribution and repeated censuses of dams along the 58-km river. The ratio fell from 19 : 1 when no beaver dams were present to <3 : 1 after 7 years of flows favourable for beaver. We investigated the dam failure-flood intensity relationship in three independent trials (experimental floods) featuring peak discharge ranging from 37 to 65 m 3 s 1 . Major damage (breach ½3-m wide) occurred at ½20% of monitored dams (n D 7-86) and a similar or higher proportion was moderately damaged. We detected neither a relationship between dam size and damage level nor a flood discharge threshold for initiating major damage. Dam constituent materials appeared to control the probability of major damage at low (attenuated) flood magnitude. We conclude that environmental flows prescribed to sustain desert riparian forest will also reduce beaver-created lentic habitat in a non-linear manner determined by both beaver dam and flood attributes. Consideration of both desirable and undesirable consequences of ecological engineering by beaver is important when optimizing environmental flows to meet ecological and socioeconomic goals. Published in

Sediment and nutrient storage in a beaver engineered wetland

“…It is likely that, as with the site studied, where closely‐spaced, multi‐dam complexes exist, these will provide a major buffering effect, reducing the likelihood of dam failure and, in so doing, also reducing the downstream release of sediment from any single dam failure. It is clear from the literature that significant uncertainty regarding dam failure dynamics exists (Anderson and Shaforth, 2010; Klimenko and Eponchintseva, 2015) and is an area in need of further research.…”

Managed Flood Effects on Beaver Pond Habitat in a Desert Riverine Ecosystem, Bill Williams River, Arizona USA

“…River stage at the RR staff gage rose 1.05 m in response to the 2008 flood pulse ( Fig. 3; see also Andersen and Shafroth 2010). Based on similarities in channel geometry, the rise was probably also~1 m immediately downstream at dams PC-I, -H, -G and -F, and perhaps -E. Below Dam PC-E, secondary channels became available, and stage rise would have been lower.…”

“…Cross-sections spanned both lentic (beaver pond) and lotic habitat at PR (n=4) and PC (n=13), but only lotic habitat at RR (n=3). A cross-section above a beaver dam was classified as being in the associated beaver pond if mean pre-flood (base flow) current velocity at that position was ≤0.2 m/s, the criterion adopted by Andersen and Shafroth (2010) to differentiate lentic from lotic reaches. We further differentiated in-pond cross-sections into those ≤20 m above the dam (Lower Pond) and those farther upstream (Upper Pond).…”

9.12 Influence of Aquatic and Semi-Aquatic Organisms on Channel Forms and Processes

“…Beaver-dam construction results in the accumulation and sorting of sediment in the upstream-ponded area, as the structure alters both the hydraulics and geometry of the channel, greatly reducing velocities and increasing the surface area (Butler, 1995;Pollock et al, 2004;Green and Westbrook, 2009;Andersen and Shafroth, 2010). Vertical aggradation rates between o0.01 m yr À1 and 0.40 m yr À1 have been reported, depending on the sediment load, hydrology, channel slope, and dam age (Butler and Malanson, 2005;Meentenmeyer and Butler, 1999;Pollock et al, 2007).…”

“…This process is then reversed with the failure of beaver dams (Andersen and Shafroth, 2010). This process is then reversed with the failure of beaver dams (Andersen and Shafroth, 2010).…”