Waiting for the snow to melt…
Beaver dams increase surface-water residence time and storage, creating conditions that can enhance evaporative water loss and alter the stable isotope composition of stream water. During the anomalously low-snowpack 2026 season, I investigated natural variation in stable water isotopes (δ¹⁸O and δ²H) across three headwater streams in Gunnison County, Colorado to evaluate evaporative fractionation associated with beaver activity with the purpose of identifying signals of evaporative water loss. The three streams occupy climatically similar subalpine watersheds but differ in bedrock geology, restoration history, and degree of beaver influence: Trail Creek, a young beaver system re-established through a 2021 restoration effort; Coal Creek, an older, naturally established beaver system; and Italian Creek, an undammed control. Water samples were collected from upstream and downstream reaches of beaver dammed creeks across the sampling season and analyzed for δ¹⁸O and δ²H using cavity ring-down spectroscopy, and continuous gauge records at Trail Creek were used to develop a stage-discharge rating curve. Lc-excess declined significantly downstream at Trail Creek across all sampling dates, tracking cumulative beaver dam count and dam length, while Coal Creek showed a weaker, directionally consistent pattern. The undammed Italian Creek control showed little spatial change. A linear mixed-effects model confirmed a significant upstream-downstream enrichment effect that varied in magnitude across the three watersheds, indicating that beaver damming, rather than distance alone, drives a negative drift in lc-excess, though its strength differs by site. At Trail Creek, lc-excess was strongly correlated with discharge, particularly downstream of beaver influence, and became significantly more enriched as streamflow declined. These results indicate that beaver ponds measurably increase evaporative water loss, with the strongest effect where dam density and residence time are greatest. Evaporative loss to streams is relevant to beaver dam analog (BDA) restoration because their structures do more than store water. They create a complex balance of net water loss and gains across the watershed.
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