Quantifying form resistance is essential for estimating summer low and bankfull flow from stream survey channel morphology

Reliable estimates of low flow and flood discharge at ungaged locations are required for evaluating stream flow alteration, designing culverts and stream crossings, and for interpreting regional surveys of habitat and biotic condition.  The majority of locations where discharge information is lacking are small, remote streams that are ungaged; and these streams typically have complex channel morphology.  Complex channels typically contain large scale hydraulic roughness elements that make it difficult to measure channel cross-section area and water velocity, or to measure channel volume where discharge is known.  In channels with large channel form roughness, it is equally difficult to estimate discharge from slope and channel dimensions using commonly applied equations. We employed a novel approach that explicitly accounts for hydraulic resistance from large wood and riffle-pool morphology in calculating low flow and bankfull discharge from stream and river physical habitat data collected as part of the US Environmental Protection Agency’s National Aquatic Resource Surveys (NARS).  Log-transformed morphology-based estimates of summer discharge (low) flows and direct field measurements of discharge in 2333 wadeable streams in the conterminous US (CONUS) showed reasonable agreement (R2 = 58%, RMSE=0.647; RMSD=0.693) for discharges ranging from 0.000036 to 100 m3/s (0.0013 to 4300 cfs) and drainage areas of 0.1 to 196,000 km2.  For a subset of 759 of 777 larger wadeable streams and non-wadeable rivers within the a greater set of 4,229 NRSA sites (drainage areas 5.5 to 2,880,000 km2) across the CONUS where nearby USGS gage data was available and adequate, our morphology-based discharge estimates for the day of summer sampling agreed fairly well (Log-transformed data; R2 = 70%, RMSE = 0.561; RMSD = 0.576) with USGS 20 yr average August mean flows ranging from 0.003 to 16,000 m3/s (0.1 to 560,000 cfs).  Similarly, morphology-based estimates of bankfull flow ranging from 0.3 to 100,000 m3/s (10 to 3,500,000 cfs) with the 1.5-year recurrence interval flood in these gaged sites (Log-transformed data; R2=0.60, RMSE = 0.582; RMSD = 0.582). Our findings  demonstrate the importance of quantifying large-scale form resistance in natural stream channels.  Our novel approach will enable examination of discharge and its ecological influence across the full range of stream and river sizes sampled by NARS or other synoptic surveys where comprehensive measures of biota, physical habitat, and chemistry are also made. Although these estimates exhibit some variability, they are adequate for examining regional patterns in discharge and flow alteration and their association with instream biota and anthropogenic disturbances. These morphology-based estimates provide summer low and bankfull flow estimates where reliable estimates are lacking. They also provide an independent validation of GIS-based model discharges (e.g., NHDPlusV2.1), and could be used to improve estimates of discharge at unsampled or ungaged locations.

Impact/Purpose

Flow alterations are a major cause of habitat degradation and impairment of water quality and biotic integrity in rivers and streams of the U.S. and worldwide.  In the conterminous United States (CONUS) flow alterations result from impoundments, irrigation withdrawal and return flows, groundwater extraction, and climate-related changes in runoff magnitude and timing. Unfortunately, because long-term daily flow measurements are limited to gaged locations, it is currently not possible to assess hydro-alteration at the vast majority of CONUS stream and river segments, which are ungaged.  The U.S. EPA’s National Aquatic Resource Surveys (NARS) assess the condition of biotic assemblages and physical habitat structure in streams and rivers of the U.S., but do not yet have site-specific indicators to monitor the status and trends in the magnitude of discharge.  NARS field crews routinely measure discharge on wadeable stream sample sites on the low flow summer days on which they were sampled.  However, NARS does not make direct field discharge measurements on the larger (boatable) sample sites, nor are they made on any rivers and streams during bankfull flows.  Furthermore, even in wadeable streams and rivers, attempts to measure discharge directly are frequently unsuccessful in shallow water and small or complex stream channels.  Available modelled flow estimates are in several ways insufficient for assessing hydrologic conditions at ungaged locations.  Annual and monthly discharge estimates for every CONUS stream segment are available from the National Hydrography Dataset version 2.1 and other sources.  Unfortunately, those estimates are based on models using geospatially derived data at USGS gaging sites, which are known not to be representative of all watersheds, particularly for small streams and those in arid areas. As a result, discharge estimates have unknown—and likely large—uncertainty in many geographic areas and almost universally in headwaters. As a consequence, flows to which biota and the stream channel are exposed, are unavailable for most streams in the CONUS. Evaluation of the effect of streamflow and its alteration on the physical, chemical, and biological condition of flowing waters and the ecosystem services they provide is a high priority for NARS and science in general. The basic data for that evaluation is lacking.  The authors developed a practical solution to this dilemma by calculating discharge from field measurements of slope, channel dimensions, and channel roughness elements collected using  NARS Physical Habitat methods.  In comparisons with measured and gaged discharges available in a subset of NARS sites, they demonstrated good agreement between their calculated discharges and measured or gaged discharge at low flow and bankfull flood stages. These morphology-based estimates provide flow information at locations and times where landscape-level models like NHDPlusV2.1 fail to do so, and could also be used to improve GIS-based estimates of discharge at unsampled or ungaged locations.  

Citation

Kaufmann, P., D. Carlisle, J. Faustini, M. Weber, A. Herlihy, R. Hill, A. Kasprak, AND S. Paulsen. Quantifying form resistance is essential for estimating summer low and bankfull flow from stream survey channel morphology. Elsevier BV, AMSTERDAM, NETHERLANDS, 466:109360, (2024). [DOI: 10.1016/j.geomorph.2024.109360]

DOI: Quantifying form resistance is essential for estimating summer low and bankfull flow from stream survey channel morphology