Downstream effects of novel water-injection dredging experiments on macroinvertebrate assemblages of a Great Plains River

Abstract

Sediment accumulation threatens the long-term viability of reservoirs worldwide. By trapping upstream sediments, reservoirs progressively lose water storage capacity while depriving downstream ecosystems of ecologically important sediment inputs. These challenges have driven interest in sediment management strategies that address both storage loss and downstream sediment connectivity. Water-injection dredging (WID) is an emerging reservoir-management technique that resuspends sediment from the reservoir bed and transports it downstream through dam outlets. However, the effects of reservoir sediment releases on downstream river ecosystems remain poorly understood. WID was conducted in Tuttle Creek Reservoir during fall 2025 and spring 2026, representing a novel application of WID in a large reservoir. To evaluate the effects of increased suspended and deposited sediments downstream of the reservoir, we quantified macroinvertebrate responses to WID relative to site-specific baseline conditions. We hypothesized that WID would negatively affect macroinvertebrate communities, with the strongest effects occurring near the dam attenuating with downstream distance. Macroinvertebrates were collected monthly using one-hour multihabitat D-net surveys from spring, summer, and fall 2023 to 2026 to characterize pre- and post-WID communities. Our main analytical approach was to evaluate if post-WID samples fell within site- specific baseline conditions using multivariate ordinations, log-response ratios and rarefaction analyses of taxonomic richness. WID produced a measurable downstream sediment signal, with turbidity increasing by as much as 20-fold relative to baseline conditions and elevated suspended sediment detected up to 198 km downstream. Fine sediment deposition occurred at all downstream sites but was greatest in the Big Blue River and was concentrated along shorelines and other low-velocity habitats. Macroinvertebrate responses differed between demonstrations and were most pronounced three days after spring WID, when richness declined at most sites and taxonomic and functional composition shifted outside site-specific baseline conditions. These compositional changes were associated partly with increased representation of planktonic taxa. In contrast, responses following fall WID were weaker and limited to fewer sites. Responses coincided with substantial hydrologic and seasonal variability, limiting attribution to WID alone. Because both WID demonstrations used controlled releases and produced suspended-sediment concentrations within the river’s natural range, the observed responses reflect moderate operating conditions and may not represent more intensive WID applications. Overall, WID coincided with measurable short-term responses, but macroinvertebrate effects differed between demonstrations.