INFLUENCE OF NATURAL BEAVER DAMS ON GROUNDWATER RECHARGE DYNAMICS IN A STREAM-ALLUVIAL-BEDROCK AQUIFER SYSTEM

Seepage losses from streams represent an important source of groundwater recharge for many aquifers. This study evaluated how seepage recharge is influenced by beaver dams on two perennial streams that overlie the Denver Basin Aquifer System in Colorado. Detailed analysis of historical imagery for one stream confirmed a high degree of transience in surface water coverage associated with beaver ponds. Along a 0.65-km study reach, the extent of surface water varied between 1,880 m² to 15,900 m² during a 19-year period from 2002 to 2021. Streambed piezometer nests installed as part of a long-term monitoring project captured the influence of dam construction and removal. Sites upstream of dams recorded rapid increases in stage (< a few days) accompanied by increased seepage (downwelling) of up to 9x pre-dam rates. Downstream sites recorded rises in deeper groundwater levels and increased upwelling of up to 10x pre-dam rates. The sites gradually returned to pre-dam conditions over several months. Groundwater flow modeling illustrates how the dams affect groundwater recharge patterns and fluxes. The study reaches occur within a stream-alluvium-bedrock sequence with a relatively shallow (variable depths < 15 m) sedimentary bedrock aquifer. Surficial beaver ponds alter deeper flow paths that produce bedrock aquifer recharge.