Oral Presentation Society for Freshwater Science 2025 Annual Meeting

Tradeoffs with hydrologic and direct measurement techniques to assess average stream depth over a defined length of stream (118378)

Walter Dodds 1 , William H. McDowell 2 , Eugenia Marti 3 , William Bowden 4 , Jennifer Tank 5 , Daniel Sobota 6 , Robert Hall 7 , Geoffrey Poole 8 , Abagail Langston 9
  1. Kansas State University, Manhattan, KS, United States
  2. University of New Hampshire, Durham
  3. Centre d’Estudis Avançats de Blanes, CEAB-CSIC, Blanes, Spain
  4. Rubenstein School of Environment and Natural Resources, University of Vermont, Burlington, Vermont, United States
  5. Biological Sciences, University of Notre Dame, Notre Dame, Indiana
  6. Oregon Department of Environmental Quality, Portland, Oregon
  7. Flathead Lake Biological Station, University of Montana, Polson, Montana
  8. Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana
  9. Kansas State University, Manhattan, KS, US

Stream depth strongly influences physical, chemical, and biological processes.  Spatial heterogeneity in stream depth varies dramatically with stream geomorphology and discharge. This variability could make extrapolation to the entire stream and river reach difficult. Here we compare two common methods used to provide a stream reach-average measurement of stream depth. The first uses a solute release with a non-reactive conservative tracer. This hydrologic method yields simultaneous measurements of average stream velocity and discharge along a defined length of stream by analyzing a continuous or pulsed solute addition, and allows estimation of average depth across an entire reach when combined with measures of average wetted channel width. The second method uses direct measurements of depth with meter sticks (metered method) and is more commonly adopted. The  metered method uses multiple transects with evenly spaced point measurements of depth with a measuring stick. We compared these two methods in 72 stream reaches of around 100 m length (approximately 20 minute average water travel times). Streams encompassed a wide array of sediment types from sealed concrete bottom channels to those dominated by boulders, and included urban, agricultural, and native habitats. We avoided complex channels (e.g. braided or those with tributaries).   The ratio of hydrologic to metered depth varied widely (from 0.12 to 5.8). Thus, the discrepancy between the methods in a particular stream could be large. The metered method generally gave greater depth than the hydrologic method at discharges less than 2 L/s. While we did 10-15, 10 depth transects, modeling our data suggests around 60 transects (600 depth measurements) are necessary to constrain depth estimates. Researchers should carefully consider why they are measuring average stream depth and logistical constraints before they choose direct measurement or solute release methods.