Beaver dams, sediment dynamics and morphological change, Odell Creek, southwest Montana

Beaver (Castor canadensis) were historically part of riverine systems across North America, and enhancement of beaver populations is increasingly considered an important remedy for stream degradation problems such as incised channels. However, how beaver affect fluvial processes and resulting morphology in different fluvial environments and on various channel types requires further attention. We examine the effects of beaver damming on Odell Creek, a relatively high-energy piedmont stream in the upper Missouri River basin of southwest Montana, where air photo and real-time observations indicate that main-channel dams typically persist for only a few years. Odell Creek has a basin area of 46 km2, a snowmelt-dominated hydrograph, and peak flows of 2-10 m3s-1. Odell Creek is broadly incised along most of its length within a late Pleistocene fluvial fan surface, with mean floodplain width between confining terraces of 240 m. Channel gradient declines downstream from 0.018 - 0.004, and mean channel width for 46 cross-sections is 8.1 m. We examined the geomorphic effects of active beaver dams and the persistence of dam-induced changes in nine study reaches representing downstream channel variability and variations in dam history. In-channel sediment characteristics and storage were investigated using pebble counts, fine sediment surveys and bed sediment mapping. Discharges exceeding bankfull during 2011 spring runoff breached three active dams within reaches surveyed in 2009 and 2010, allowing for repeat channel cross-section and sediment surveys. Channel geometry and sediment analyses were also conducted at several other active and breached dam sites. Volumes of fine (≤ 2 mm) sediment stored upstream of active beaver dams ranged from 40 - 135 m3. Observations and surveys of abandoned dam sites and dam breaches revealed that the majority of sediment stored upstream of beaver dams is quickly evacuated following a breach. However, while general aggradation from damming was not observed, small volumes of sediment may remain in storage on elevated gravel bars and on floodplains above bankfull levels. At one site, beaver damming increased floodplain elevation by ~ 20 cm. Persistence of morphological changes induced by beaver dams appears dependent on the style of dam breaching. Where breaching of the entire dam occurs, net morphological change is minimal. Where a breach occurs that leaves part of the dam intact, eddies downstream of the dam remnant promote fine-grained sediment storage, whereas bank erosion and collapse commonly occur directly adjacent to the breach, locally increasing sinuosity. At other sites, dams were a contributing factor in channel avulsions and creation of meander cutoffs. Transient beaver dams on Odell Creek appear to play a role similar to woody debris, introducing cross-channel structures and promoting channel complexity. In-channel sediment storage in the main Odell Creek channel is limited by frequent breaching (<1 - 5 years). However, damming promotes floodplain storage and breaching induces channel morphological change. Dam breach frequency and breach style contribute to the type and longevity of morphological change and sediment storage associated with beaver damming.