Stream hydromorphology regulates in-stream water flow and interstitial flow of water within streambed sediments, the latter known as hyporheic exchange. Whereas hyporheic flow has been studied in sand-bedded streams with ripples and dunes and in gravel-bedded streams with pool-riffle morphology, little is known about its characteristics in plane bed morphology with subdued streambed undulations and sparse macroroughness elements such as boulders and cobbles. Here, we present a proofof-concept investigation on the role of boulder-induced morphological changes on hyporheic flows based on coupling large-scale flume sediment transport experiments with computational fluid dynamics. Our results show that placement of boulders on plane beds increase the reach-scale hyporheic median residence time, τ 50 , by 15% and downwelling flux, q d , by 18% from the plane bed. However, reach-scale hyporheic exchange changes are stronger with τ 50 decreasing by 20% and q d increasing by 79% once the streambed morphology reached equilibrium (with the imposed upstream sediment and flow inputs on boulders). These results suggest that hyporheic flow is sensitive to the geomorphic response from bed topography and sediment transport in gravel-bedded streams, a process that has been overlooked in previous work.

This is the second of three reports in a multichapter volume characterizing water resources in the Big Lost River Basin. These reports document the findings of a hydrogeologic investigation of the Big Lost River Basin that was jointly conducted by the U.S. Geological Survey, Idaho Department of Water Resources, and the Idaho Geological Survey from 2018 through 2021. Chapter A (Zinsser, 2021) describes the hydrogeologic framework of the Big Lost River Basin. It includes a conceptual definition of the hydrogeologic units, a three-dimensional hydrogeologicframework model, and a description of groundwater occurrence and movement. Chapter B (this report) describes streamflow gains from and losses to groundwater in the Big Lost River between Mackay Reservoir and south of Arco, Idaho. Streamflow gains and losses were estimated from a series of four surface-water measurement events during pre-and post-irrigation season conditions from 2019 to 2021. Chapter C will describe groundwater budgets for the Big Lost River Basin from 2000 to 2019. The groundwater budgets will provide annual estimates for aquifer inflows and outflows. Collectively, these reports present a characterization of water resources in the Big Lost River Basin that will help address current challenges in water-resource management.