GEOMORPHIC RESPONSES TO POST-GRAZING RECOVERY AND STREAM RESTORATION IN SEMIARID GRASSLAND STREAMS

Abstract

Semiarid grassland streams are highly sensitive to land use, climate, extreme discharges, and internal geomorphic thresholds that drive episodic erosion. Rooted in a process-based philosophy and commonly applied to historically wood-rich, beaver-modified systems, the application of low-tech process-based restoration through structures such as post-assisted log structures, beaver dam analogs, and rock-based designs is increasingly being extended to other geomorphic settings, including semiarid grasslands. In these wood-poor systems with fine-grained substrate, channel adjustment depends less on wood-mediated complexity or interactions with large clasts and more on subsurface soil piping and asynchronous responses to changing boundary conditions. Here, I evaluate post-grazing recovery and structural restoration outcomes at transect-to-landscape scales in two Southern Plains Land Trust preserves in southeastern Colorado. Following grazing cessation, soil compaction decreased after roughly a decade of rest, as indicated by a multivariate linear regression analysis of 238 soil bulk density samples. In contrast, visible above-ground upland vegetation appears to recover on timescales longer than the observation window (>10 years), based on analyses of growing-season composite Landsat imagery from 2013–2024. Logistic regression models indicate that the performance of restoration structures depends strongly on drainage area, slope, local vegetation cover, and the recency of large-scale commercial grazing. In total, only ~30% of the 92 in-channel structures evaluated (mostly ≤4 years old) were both intact and produced visible geomorphic change. Structures in piping-dominated headwaters remained intact but rarely trapped sediment, whereas those in larger catchments were more likely to initiate local geomorphic change, despite having higher odds of destruction during peak flows. Simple numerical modeling suggests that changes in soil surface macroporosity—driven by land-use impacts on vegetation root communities, enhanced desiccation cracking, or shifts in burrowing animal populations—can influence water delivery to perched saturated layers and the initiation of soil piping. Collectively, these findings highlight the need to match restoration strategies with the geomorphic processes driving channel adjustment and call into question the extent to which commonly implemented structure-based interventions in grassland systems can be considered truly process-based.