Highlights

The Global Sediment Cycle

From Source to Sink

Mapped the global distribution of Earth's sediment sources and sinks as step towards developing geomorphology as a global science (194).

Soil Production

Showed that the rate of soil production rate—the conversion of rock to sediment—is paced by rock fracturing rather than the canonical view of weathering mediated by soil thickness (198).

River Erosion into Rock

Developed and tested theory for how rivers erode into bedrock from wear by sand blasting (16, 39, 53, 64, 168).

Tracking Landslides from Remote Sensing

Developed an inverse model that can be used to calculate landslide thickness and rheology from remotely sensed elevation data (45), and investigated landslide speeds and triggers in California and France as well as in analog experiments (27, 45, 91, 96, 120).

Wildfires and Debris Flows

Working in Southern California, we developed and tested theory for, and showed the dominance of, dry ravel—the sloughing of soil and sediment from steep hillslopes—during and immediately following wildfire before any rainfall (26, 40, 83). Repeat lidar and field monitoring observations reveal that dry ravel loading fuels post-wildfire debris flows (117, 132).

Sediment transport in mountain streams

Explained the counterintuitive observation that boulders become increasingly difficult to move in steeper mountain streams, and identified the transition to debris flow generation at very steep slopes using novel flume experiments, field monitoring and theory development (15, 37, 42, 48, 61, 62, 63, 82, 86, 88, 97, 102, 195).

Grainsize Gap

Explained a long standing peculiarity of missing sediment with diameters of 1-10 mm on Earth's river beds (72, 123, 173, 186).

Mud transport

Discovered the ubiquity of flocculation of mud in freshwater rivers and deltas and its importance for carbon transport and land building, including theory development, laboratory experiments, and field campaigns in the Mississippi Delta and Yukon Rivers (118, 121, 122, 130, 149, 151, 164, 184, 187, 196).

Seismic geomorphology

Developed the first theories for seismic noise generated by river water, sediment transport and debris flows to use for monitoring in mountain rivers, including application to the deadly Montecito debris flows in 2018 (32, 58, 98, 105, 113).

Landscape Evolution

Self-formed waterfalls

Discovered how bedrock waterfalls can emerge from a planar bed and propagate by plunge pool drilling, controlling the pace of landscape change at field sites in the San Gabriel Mountains and Hawaii (11, 49, 56, 67, 71, 87, 88, 90, 108, 138).

Megafloods and Bedrock Canyons

Showed how classic work had overestimated the largest known floods on Earth and Mars, and how these floods can be so efficient at forming canyons by plucking erosion, including field sites across Idaho, Texas, Washington and Mars (6, 9, 17, 18, 21, 47, 75, 79, 93, 114, 137, 148, 156, 179).

River Valley and Terrace Formation

Explained why some bedrock river valleys are wide with terraces while others form narrow slot canyons as a result of non-linear interactions between river lateral migration and bank bedrock content (46, 52, 70).

Riverbank Erosion in Permafrost

Developed and tested thermal-mechanical theory for river erosion in permafrost in novel "ice rink" flume experiments; showed competing effects of how permafrost thaw is speeding up river erosion, whereas sediment entrainment and less violent ice breakup is slowing it down with sub-pixel bank detection from remote sensing, and major field campaigns in the Yukon and Koyukuk Rivers, Alaska (167, 169, 171, 177, 180, 183, 185, 201).

River floodplains and organic carbon storage

Developed and tested theory for river floodplain formation, storage and fluxes of organic carbon. Found that floodplain deposits are thousands of years old and vast stores of carbon through major field campaigns in Iceland and Alaska (46, 86, 119, 136, 139, 143, 150, 172, 178, 181, 192).

Deltaic avulsions

Found that the location and timing of avulsions on river deltas—the process that sets the size and channel structure of deltas and is also a major hazard—is controlled by backwater dynamics, including major field campaigns on the Yellow and Mississippi Rivers (29, 30, 31, 54, 59, 74, 77, 99, 111, 109, 116, 125, 129, 140, 141, 152, 158).

Land loss on river deltas

Investigated the mechanisms that transport sediment and build land on river deltas—avulsions, secondary channels, and flocculation—to understand where land will survive sea level rise, with application to the Yellow and Mississippi Deltas (73, 95, 126, 157, 159, 160, 166, 200, 202).

Ancient Environments of Earth and Mars

Rivers before plants

Showed how cohesive mud can provide the bank strength needed to drive meandering on Mars and early Earth before the proliferation of land plants, with field analogs in the deserts of California and Neveda. Proposed the hypothesis that it is enhanced mud flocculation driven by plant organic material that caused the fundamental change of mudstone abundance in the rock record 440 million years ago (133, 112, 115, 170, 182, 197).

Wave ripples from ancient oceans

Developed a theory and workflow to calculate ocean depths and wave conditions from measurements of ancient sandy wave ripples. This method has been used to reconstruct the sea state at critical times in Earth history, such as the Marinoan Ice Age, and infer dense atmospheric conditions on early Mars (13, 28, 33, 94, 153, 188, 191).

Crater degradation by dry processes

Used observations from NASA's Opportunity Rover to quantify the rate of crater degradation by regolith creep and aeolian infill, and showed how bedrock chutes and gullies can be explained by dry rock avalanches, rather than flowing water as previously assumed (60, 104, 145, 146, 176, 190, 205).

Mars sedimentology from rovers

As a team member on NASA's Curiosity and Perseverance rovers, I contributed to understanding of river deposits (fluvial bars, delta bars), debris flows, rock fall, yardangs, plunging river plumes, and the origin of Mount Sharp (36, 68, 106, 147, 154, 155, 162, 174, 189, 199).

Identifying Mars' ancient coastlines

We discovered that some of Mars "inverted channels" are actually wind exhumed channel belts and delta deposits, rather than hardpan infills of tributary networks as previously assumed, using Mars observations and analog sites in Utah and Spain. This discovery changed the inferred flow direction of river networks on Mars, allowed us to recognize the depocenters and depositional rivers on Mars for the first time, map out global river networks from source to sink, and define the coastal zone of a global ocean (44, 66, 110, 127, 128, 134, 142, 144, 161, 163, 175, 193).

The unique wind ripples of Mars

We helped discover and then explain why Mars has two scales of wind ripples, whereas the Earth only has one; the larger bedform mode is suppressed under Earth's heavier atmosphere. We demonstrated how wind-ripple strata, preserved in the geologic record, can be used to find the time of atmospheric loss on Mars (76, 81, 85, 101, 135).