Projects

Lithologic controls on river response to deglaciation

Deglaciating alpine catchments represent ideal natural experiments, with large recurring pulses of sediment contributed to the fluvial network from isolated, identifiable source regions. Taking advantage of this fact, I am working to integrate field and remotely sensed data with coupled hydrologic and morphodynamic models to compare basins where rapidly retreating glaciers of similar size carve highly resistant and comparably friable bedrock.

This work is supported by NSF PREEVENTS, a 5-year collaborative research project focused on a mountains-to-coastline approach to flood hazard prediction lead by Erkan Istanbulluoglu (UW CEE). The collaborators on the project are numerous, but I'm working closely with Brian Collins (co-PI, UW ESS), David Shean (UW CEE), and Scott Anderson (USGS).

Predicting habitat via grain size

"A basin-scale method for predicting salmonid spawning habitat via grain size and riffle spacing, tested in a CA coastal drainage"  

Paper: ESPL, 2016

We have developed a physically-based, empirically calibrated approach to predicting grain size distributions from high resolution LiDAR (Light Detection and Ranging)-derived topographic data for a 77 km2 watershed along the central California Coast. This approach builds on previous efforts in that it predicts the full grain size distribution and incorporates an empirically calibrated shear stress partitioning factor. Predicted grain size distributions are used to calculate the fraction of the bed area movable by spawning fish. We then compare the ‘movable fraction’ to 7 years of spawning survey data. We find that movable fraction explains the paucity of spawning in the upper reaches of the drainage, but does not explain variation within the mainstem.

Sediment supply and the above-threshold channel

"Sediment supply controls equilibrium channel geometry in gravel rivers"

Paper: PNAS, 2017

Press: Phys.org

           Santa Cruz Sentinel

Gravel-bedded river channels commonly evolve so that median sized grains on the bed surface begin to move when flow is at, or near, bankfull flood stage. However, not all natural gravel channels conform to this simple relationship; some channels maintain bankfull stresses far in excess of what is needed to initiate sediment motion. We use a database of >300 gravel-bedded rivers in North America to show that continent-wide trends in observed bedload transport capacity parallel trends in observed erosion rates. Notably, we find that the ratio of bankfull to critical Shields stresses is significantly higher in West Coast river reaches (2.4, n=77) than in river reaches in the rest of the continent (1.0, n=229). Using existing sediment transport models, we explore the hypothesis that these reaches have adjusted to maintain a high excess shear stress at bankfull flows to accommodate elevated sediment supplies resulting from rapid erosion along the tectonically active margin of western North America.

Regional patterns in the timing and intensity of gravel bed mobility

Paper: GRL, 2018

A simplistic view of bed mobility based on the threshold channel concept would suggest that gravel beds mobilize during bankfull flow, every ~1-2 years. However, I have combined USGS stream gage and channel morphology records and the best available sediment transport models to illuminate a far more diverse array of river bed mobility. For example, in Appalachian rivers, my modeling shows that bed mobility is rare and occurs during short duration floods that occur with little seasonal or annual consistency. In contrast, rivers on the tectonically active West Coast tend to experience bed mobility during even modest winter storms. These contrasts in bed mobility lead to diverse structural templates for river ecosystems. This framework for quantifying and characterizing river bed mobility represents an innovative new way to compare rivers.

This work has potentially important implications for both river bed “memory effects” and benthic macroinvertebrate community structure.

Fine patches among the boulders

While the drivers of grain size patch formation in low-slope rivers are well understood, there is a paucity of work on grain size patches in steep, coarse streams. What controls the spatial distribution of fine grained patches in steep streams? Are these patches an emergent characteristic of coarse, steep streams (as they are in shallower sloped rivers)?

To address these questions, I’m using exciting new tools in the field of geomorphology: bathymetric (green wavelength) LiDAR (which I obtained through a NCLAM seed grant), and structure-from-motion (SfM) photogrammetric surveys. I have refined methods to quantitatively characterize grain size patches (via SfM point cloud roughness) and channel bed morphology (via green LiDAR point clouds). Based on these data, I find that valley width is a strong control on channel morphology and gravel patch distribution in this steep, cobble- and boulder-bedded stream. Understanding controls on the distribution of gravel is important in Big Creek, which supports an endangered steelhead population that depends on the scarce gravel for spawning. This work dovetails with efforts by researchers at NMFS, who have a long-term steelhead monitoring program in the creek. My findings provide spatial context to their work on steelhead population genomics.