Predicting grain size in gravel-bedded rivers using digital elevation models: Application to three Maine watersheds

Abstract

Riverbed grain size controls suitability of spawning habitat for threatened fi sh species. Motivated by this relationship, we developed a model that uses digital elevation models (DEMs) to predict bed grain size. We tested the accuracy of our model and two existing models with channel measurements from high-resolution airborne light detection and ranging (LiDAR) DEMs. All three models assume that bed grain size is a function of reach-average high-fl ow channel hydraulics (measured by shear stress or stream power). Our test data are fi eld measurements of median grain size (D 50 ) at 276 stations along four rivers in Maine. Pleistocene continental glaciation strongly infl uences the longitudinal profi les, which have alternating steep and gradual segments. We exploit the resulting variations in sediment supply to understand the controls on model success or failure in predicting bed grain size. Results show that all three models have ~70% success in predicting D 50 within a factor of two overall, and better where the rivers are coarse gravel bedded (~80% success where D 50 ≥ 16 mm). This similarity is unsurprising given that the models primarily rely on channel gradient (S) and drainage area as inputs. Measurements of S from LiDAR DEMs yield only a modest improvement in model success over those from topographic maps. We fi nd that our model works best in sediment-starved steep reaches. Model failures fall into two broad cate gories:(1) relatively fine-grained (D 50 < 16 mm) depo sitional reaches where our assumption of a constant, bankfull threshold for bed mobilization may be invalid; and (2) reaches where local variations in hydraulic roughness and/or sediment supply control D 50 . We argue that models based on airborne infrared LiDAR DEMs may reach a maximum around 80%-85% accuracy due to these sub-reach-scale factors, which cannot be easily measured from DEMs. The overall success of the models in predicting grain size indicates that the morphology of these channels has adjusted to the imposed S and sediment load during the ~15 k.y. since deglaciation and through the period of anthropogenic channel change over the past three centuries.

Basin‐scale methods for predicting salmonid spawning habitat via grain size and riffle spacing, tested in a California coastal drainage

“…Using HEC‐RAS allowed us to make >1500 width estimates throughout the drainage (which we also use to estimate riffle density, below), a task that would be cumbersome by hand. Following the method described by Snyder et al (), we used these point measurements to calibrate a basin‐wide hydraulic geometry scaling relationship (Leopold and Maddock, ) to relate w (m) and A (m 2 ) (Supporting Information, Figure S1(b)): …”

“…Based on this finding, we adjusted our Scott Creek Q 2 to be 9% higher than that from the years on record. Using this adjustment and a linear scaling between discharge (m 3 /s) and drainage area (m 2 ) (Snyder et al, ) the bankfull discharge to drainage area relationship is: …”

Performance evaluation of three DEM‐based fluvial terrace mapping methods

“…We have conducted extensive field-based research in the watershed (576 km 2 ) in recent years (e.g. Snyder et al, 2008;Snyder et al, 2013;Strouse, 2013;Hopkins, 2014). The river has a mean gradient of 0.0016.…”

Mapping spatial patterns of stream power and channel change along a gravel-bed river in northern Yellowstone

“…This study applied the same value, the median of D 50 values from Wolman pebble counts conducted throughout the study area, to all budget cells, but in reality variations in particle size along Soda Butte Creek would have dictated where ω exceeded ω c and subsequently affected our calculations of ε. A more rigorous assessment could use a DEM to predict alongchannel changes in median bed grain size, following the approach of Snyder et al (2013), and ideally would quantify the uncertainty associated with these predictions by comparing estimated…”