The Limits to Flow Impacts from Beaver-based Restoration in Headwater Streams

Joel Sholtes, PhD, PE1 

Beavered stream corridors influence the rate, magnitude, and timing of water and sediment movement through a watershed. Low-tech process-based restoration in headwater streams often has the goal of restoring these processes in incised stream corridors via construction of beavery mimicry structures and/or recolonization by beaver. Stream corridors with beaver dam-pond complexes have demonstrated the ability to attenuate flood peaks and detain sediment. Impacts on non-peak stream flows (e.g., summer and fall base flows) are less straightforward.  Empirical studies on beaver and beaver mimicry impacts to non-peak flows have yielded mixed results. Modeling studies have lent to a physical understanding of how beaver and beaver mimicry could lead to base flow impacts. A hydrologic and sediment monitoring program established in 2023 is evaluating surface and subsurface hydrologic and sediment response to stream rehabilitation projects across the State of Colorado whose goal is beaver recolonization, supported with beaver mimicry structures. 

Six sites slated for beaver mimicry treatments in 2025 – 2026 have been instrumented with stream gaging stations bracketing the treatment reaches and groundwater elevation monitoring transects. Pre-treatment, baseline data collected in 2023 and 2024 demonstrates that reach-scale water balance is typically net positive or gaining (Qout – Qin > 0) during the spring runoff and early summer. Then, depending on the magnitude of hillslope or tributary water inputs, as well as the depth of a confining layer, these streams typically can become losing (Qout – Qin < 0) later in the summer and fall as the water table falls below the stream surface. Summer rain events can temporarily reverse this trend as can the end of the growing season when vegetation becomes dormant. After treatment, streamflow is temporarily reduced (hours to days) as the flow is detained behind installed structures. New or restored surface flow paths and detention behind in-channel structures contribute to an increase in ground water elevation. How this translates to streamflow is a function of the quantity of water detained in the surface and subsurface, the hydraulic head (elevation difference) between this water and a surface outflow point, and the distance and time of this water from a surface outflow point.