Research Highlights
Our research is focused in three major themes: 1) the global sediment cycle, 2) landscape evolution, and 3) reconstructing ancient environments on Earth and Mars. Research highlights from our past work are listed below.
The Global Sediment Cycle
Earth’s surface topography is constantly changing due to the sediment cycle—the processes that erode rock in mountains to produce sediment, transport sediment downstream through river networks, and deposit that sediment to build new land in floodplains and coastal wetlands. Over years to decades, these sediment-movement events can impact people, infrastructure, ecosystems and global carbon cycle. Our work has contributed to new understanding of sediment transport processes from their sources to their sinks.
From Source to Sink
Mapped the global distribution of Earth's sediment sources and sinks (194).
Soil Production
Showed that the rate of soil production—the conversion of rock to sediment—is paced by rock fracturing rather than the canonical view of weathering mediated by soil thickness (198).
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).
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).
Landscape Evolution
The sediment transport processes discussed above shape the landscapes we live on from mountain tops to coastal plains. Our research has contributed to quantifying the rates and mechanisms of landform development. Some landforms develop over thousands or even millions of years, requiring the sleuthing skills of geology. We also study hotspots where today change is happening at unprecedented rates such as thawing permafrost river plains in the Arctic, drowning coastlines of the Mississippi Delta, and the consequences of those changes on people, ecosystems and the carbon cycle.
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 blocks. We worked on important field examples across Idaho, Texas, Washington and Mars (6, 9, 17, 18, 21, 47, 75, 79, 93, 114, 137, 148, 156, 179).
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; tested these ideas using novel 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 are vast stores of carbon through major field campaigns in Iceland and Alaska (46, 86, 119, 136, 139, 143, 150, 172, 178, 181, 192).
River Avulsions on Deltas
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 field sites in the Yellow, Mississippi and Yukon Deltas (73, 95, 126, 157, 159, 160, 166, 200, 202).
Ancient Environments of Earth and Mars
What was the environment like on the surface of Earth or Mars long ago before people, plants or even life existed? The clues are written in the shape of ancient landforms and the layers of deposits now turned to rock. We study these ancient landforms and rocks to reconstruct the environmental history of Earth and Mars over geologic time.
Rivers Before Plants
Showed how cohesive mud can provide the bank strength needed to drive meandering on Mars and the early Earth before the proliferation of land plants, with field analog sites in the deserts of California and Nevada. 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
As a Science Team Member on NASA's Opportunity Rover, we quantified 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, we 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 some river networks on Mars, allowed us to recognize the depositional rivers and basins on Mars , 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 map the timing of atmospheric loss on Mars (76, 81, 85, 101, 135).