Acoustic Doppler Current Profiler Measurements

ADCPs were the primary instruments used to measure channel bathymetry and hydraulic conditions (Stephens and Siefken, 2024). ADCPs emit sound into the water column to measure water velocity via the Doppler effect (Mueller and others, 2013). The sound frequency emitted is dependent on the make and model of the ADCP, and different ADCP instruments are often designed for use in different river conditions (such as different depth, velocity, and clarity). Modern ADCP instruments can often measure velocity and direction of flow at multiple depths within the water column. These data are commonly averaged and binned over some user-defined width and depth interval.

ADCP measurements are typically made by suspending the instrument into the top of the water column and moving it across a span of the river channel in one or more passes perpendicular to the primary flow direction. Because channel hydraulic conditions are always changing, several passes are often made, and the measurements are postprocessed to produce an average of the conditions observed from each pass (Mueller and others, 2013). Additional details on methods used by the USGS to make ADCP measurements are provided in Mueller and others (2013). The USGS also uses a variety of software tools to postprocess ADCP measurement data including QRev (Mueller, 2016), RiverSurveyor LIVE (RSL; Xylem, 2023), and the Velocity Mapping Toolbox (VMT; Parsons and others, 2013). The end product is a rich dataset quantifying the shape of the channel and the spatial variation in flow velocity and direction at the channel cross section.

Cross Sections

ADCP measurements were made at the 14 permanent cross sections and 6 random cross sections. USGS station identifiers (table 1) were established for each cross section in the USGS National Water Information System (U.S. Geological Survey, 2023). The locations of the cross sections were preprogrammed as line features into a controller for a real-time kinematic global navigation satellite system (RTK–GNSS; fig. 2). This controller was then used to guide the location and direction of ADCP transect measurements at each cross section. Typical ADCP cross section measurements consisted of at least four passes and as many as six passes.

Velocity

Channel velocity characteristics are quantified and summarized using a combination of summary statistics and graphical depictions (Stephens and Siefken, 2024). The mean cross sectional water velocity (depth averaged velocity, table 1) of each surveyed cross section in the bypass channel and at the entrance and exit of the channel is determined from the QRev software as an average of all passes collected at each section. The VMT software was used to process RSL Microsoft Access table files and produce a heat map showing the average shape of the channel and spatial distribution of water velocities at each cross section (fig. 2).

The RSL Microsoft Access table files were also used to produce water velocity stick plan maps (fig. 3) in ArcGIS (Esri, 2023). These maps depict a plan view of the average velocity magnitude and direction within horizontally spaced samples at individual cross sections. Horizontal spacing is determined by applying an averaging time in the VMT Geographic Information System Export Tool. The averaging time to obtain 12–15 velocity vectors varies from survey to survey. The velocity vector sticks are useful for identifying the locations of the channel thalweg and changes in channel flow direction when compared with other surveys.

Streamflow

Streamflow (table 1) was computed for all surveyed cross sections except for those surveyed near the bypass channel entrance and exit. Streamflow was computed by postprocessing ADCP pass data in the QRev software (Mueller, 2016). The QRev software produces a final estimate of mean streamflow, as well as a measure of the uncertainty of the mean, depicted as the estimated 95-percent uncertainty. The estimated 95-percent uncertainty in the QRev software combines the random uncertainty, invalid data uncertainty, edge Q (cross section edge streamflow) uncertainty, extrapolation uncertainty, moving-bed test uncertainty, and systematic uncertainty (Mueller, 2016).

Flow Split

Flow split (table 1) was calculated as a percentage of the bypass channel streamflow to the total Yellowstone River streamflow. Total Yellowstone River streamflow was determined by the ADCP measurements collected at permanent cross section 14 at Yellowstone River mile 74 (Stephens and Siefken, 2024; fig. 3). The bypass channel streamflow was determined at the seasonal USGS streamgage located in the bypass channel (Yellowstone River fish bypass channel near Intake, Mont.; USGS station 06328495; fig. 1; U.S. Geological Survey, 2023) using the computed unit discharge value closest to the mean time of the measurement collected at Yellowstone River mile 74.

Bathymetry

The minimum depth (depth, table 1) for the deepest continuous 30-ft-wide stretch of surveyed cross section was determined from the most representative transect using the VMT Geographic Information System Export Tool with no averaging (Mueller, 2016) for each cross section. An algorithm was created using the R programming language (version 4.3.1; R Core Team, 2023) to determine the minimum depth for each 30-ft-wide section within the cross section starting with the first ADCP ensemble on the left bank. Four to six transects were collected with the ADCP at each cross section, each of which likely contains some random error; therefore, the representative transect used to determine bathymetry parameters is the transect that best represents the channel shape and velocity distribution as determined by the operator.

Water Surface Elevation

Water surface elevation (table 1) is the reference elevation for channel bathymetry measured at each cross section. This elevation is defined as the height of the water surface relative to the North American Vertical Datum of 1988. The water surface elevation for each cross section was determined by obtaining global positioning system (GPS) measurements with a survey rod along the edge of water or by using the height of the GPS antenna above the water surface elevation when using the GPS for ADCP navigation. The RTK–GNSS was used following USGS GPS survey techniques outlined in Rydlund and Densmore (2012). Control point 19, established by Reclamation, was used as vertical control for the RTK–GNSS surveys. All surveys were done using GEOID09 with Montana Federal Information Processing Standard 2500 State Plane coordinate projection.