Supporting data: Water quality, hydraulics, and aquatic plant growth in the middle Snake River, southern Idaho

The middle reach of the Snake River in south-central Idaho has a long history of water-quality impairment related to excessive total phosphorus (TP) and suspended sediment. Although TP levels appear to have declined since original total maximum daily loads were established in 1997, the Crystal Springs reach stands out with an abundance of macrophyte growth. Previous studies of macrophyte growth suggested that flow, light, nutrients, and sedimentation, or a combination of each, may control the timing and abundance of plant growth. In 2019, the U.S. Geological Survey, in cooperation with the Idaho Department of Water Resources, began a study to develop a two-dimensional hydraulic and habitat suitability model to help resource managers constrain macrophyte growth in the Crystal Springs reach. Data collected during field measurements show variability in macrophyte densities. However, initial analyses indicate that this variability cannot be entirely attributed to TP levels because they were consistently lower in this study than those observed in previous studies despite total phosphorus being a limiting factor to plant growth. Preliminary hydraulic and habitat suitability modeling results suggest that hydraulic characteristics (water depth and velocity) and flow-durations may limit or promote macrophyte growth more than TP alone. Aerial imagery data further support that higher flows limit macrophyte growth, whereas low flows promote macrophyte growth irrespective of the nutrient conditions. This data release contains the following items:
1. Volumetric Macrophyte Samples: This child item contains the results from measuring the volume of macrophytes sampled at stations along four transects within the study reach. Samples were collected on 13 different dates during the growing season (May to October) between 7/28/2020 and 8/7/2024.
2. UAS Imagery: This child item contains aerial imagery of the study area used to delineate the extent of macrophytes within the study area. Imagery was collected using uncrewed aircraft systems (UAS) on 12 different dates during the growing season between 9/5/2019 and 8/7/2024.
3. Macrophyte Delineation Shapefiles: This child item contains shapefiles describing the extent of macrophyte beds in the study reach for 16 dates, as delineated from the UAS-derived orthomosaics and historical orthophotos, between 7/25/1953 and 8/7/2024.
4. Velocity Mapping: This child item contains depth-averaged velocities measured in six transects within the study reach on 9 dates between 6/23/2020 and 9/30/2021. Measured velocities supported the development and verification of the two-dimensional hydraulic model.
5. Two-dimensional Hydraulic Model Archive: This child item documents the two-dimensional hydraulic model used to simulate flow depth and depth-averaged velocities within the study reach for three periods with different extents of macrophyte growth in the study reach. Water years 1996-2000 were characterized by generally low macrophyte growth while water years 2001-05 and 2019-22 were characterized by extensive macrophyte growth. Model simulations result in 50Gb of result exports. As a result, the model archive does not contain results but instead provides the model template and other required files for users to run simulations and export the results.
6. Other Data: This child item contains data related to drifting macrophyte accumulations, external phosphorus and nitrogen data, nutrient growth assays, sediment nutrient data, and light extinction data.