Author(s): Yuiko Chino; Gregory Pasternack; Takashi Gomi; Shinji Fukuda

Linked Author(s): Gregory Pasternack, Shinji Fukuda

Keywords: Ecohydraulics; Habitat; Instream flows; Hydrodynamic modeling; Mountain rivers

Abstract: Microhabitat statistical analysis remains the primary scientific and management basis for instream flow studies using ecohydraulic modeling. The wide adoption of spatially explicit habitat mapping and modeling enables consideration of patterns of microhabitat patches and their potential importance for fluvial ecological functions. Moreover, patch spatial patterns likely change with discharge, yet no framework for investigating this exists. In this study, new concepts and methods were developed to characterize spatial patterns of microhabitat patches (a.k.a. mesohabitat) and discharge-dependent patch behaviors. The five types of patch behaviors were disappearance, expansion, contraction, aggregation, and segmentation. Patch characterization and behavior analyses were applied to adult rainbow trout habitat along ~ 12 km of confined mountain river in the Sierra Mountains of California. Expert-based rainbow trout depth and velocity habitat suitability curves were coupled with meter-resolution 2D hydrodynamic modeling to identify high-quality microhabitat patches at 7 discharges ranging from baseflow to modest flooding. Area of high-quality microhabitat, patch count, and patch size were analyzed for each discharge. Patch behaviors were tallied for 56 patches among 5 patch size classes (0.25-2.5, 2.5-25, 25-250, 250-2500, and ≥ 2500 m²) for each discharge increase between the 7 flows, except only 46 patches were analyzed for the highest flow increment. Simulated high-quality microhabitat area varied between 22,536.50-76,1598.75 m² depending on discharge. Three different regimes in flow-dependent patch size change were found along with a gradual decrease in patch persistence with discharge. The occurrence of patch behaviors had a trend explaining patch size and persistence change. When the flow increased from 5.38 m³/s to 9.91 m³/s, about 30% of microhabitat patches was estimated to disappear. Expansion and aggregation behavior had the largest count at the flow increment ranging from 0.14 to 0.42 m³/s, and decreased as discharge increased. On the other hand, contraction and segmentation behaviors had the minimum count at the flow increment of the baseflows and 0.42 m³/s, and increased with the increasing in flow. These trends were consistent with the result of patch metrics calculation and the observation of the flow-dependent microhabitat patch dynamics. Some behaviors indicated the emergence of high-quality patches in flood flow and the occurrences appeared related to geomorphic features. Overall, analysis of patch behavior dynamics enriches ecohydraulics and should be considered for use in river management.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022347

Year: 2022