Research

River Response to the Removal of a Small Dam

Small dams are scattered throughout many coastal watersheds along the west coast of the United States. Often built in the middle of the last century, most of these dams are now full of sediment and no longer serve their original purpose. Moreover, these structures obstruct the upstream passage of anadromous fish, such as coho salmon and rainbow trout, to their historical spawning grounds and obstruct the downstream passage of spawning gravels needed by these fish to build their nests. As a result, there is a growing movement to remove these dams to try to restore the native fish populations. Little is known, however, about how and how quickly rivers respond to the removal of these structures. To address this problem, my research group has been monitoring a creek, near Santa Cruz (CA), which had its dam removed in October 2021. To see a time-lapse of the dam removal, click here.

Mill Creek Dam. Photo credit: Ian Bornarth

Dam gone.

Cenozoic History of the Sierra Nevada

Since the early 2010's, I've been interested in understanding the history of the Sierra Nevada over the past 60 million years. The dominant paradigm claims that this range experienced significant uplift over the past 5 million years. However, my investigations have shown that much of the evidence used to support this claim is fundamentally flawed, particularly with respect to the northern half of the range. Although these results have been controversial, they are supported by isotope studies. Below, I provide summaries of papers that I've written on the topic.

Topographic Inversion in the Stanislaus Watershed? Not Really

One of the first lines of evidence used as support for significant Late Cenozoic uplift of the Sierra Nevada was the presumed inversion of the bedrock landscape in the Stanislaus River watershed. This idea was based on a geological cross-section that showed a buried valley underneath volcanic rocks that cap a modern-day flat-topped ridge (image to the left). According to this theory, the lava flow had followed a bedrock valley about 10 million years ago and, since then, the ridges of the valley had eroded down, leaving the former valley bottom as a topographic high point. Given the high erosion rates that would have been necessary to accomplish this inversion of the landscape, it was assumed that it was driven by significant uplift of the Sierra Nevada. To test this hypothesis, I looked for evidence that the flow had followed a bedrock valley and found none. Instead, the flow followed a depression cut into older sediments that had buried the bedrock landscape. In addition, based on the disposition of these older sediments, it appears that the bedrock landscape has changed little over the latter half of the Cenozoic. Finally, I discovered that the geological cross-section that underpinned the theory of topographic inversion was fictional. The conclusion of this study, published in Geological Society of America Bulletin, is that there is no evidence in the Stanislaus River watershed for significant late Cenozoic uplift.

Large Swaths of the Northern Sierra Nevada Buried by Gravels in the Eocene

The ancient auriferous gravels that helped spawn the California Gold Rush have figured prominently in investigations of the Cenozoic history of the Sierra Nevada. These fluvial sediments, scattered throughout the northern half of the range, are the remnants of much larger deposits that accumulated throughout the Eocene and Early Oligocene. In this study, first-authored by my MS student Christina Tipp, we present a reconstruction of the original extent of the gravels developed according to a few simple rules. This reconstruction suggests that large swaths of the northern Sierra Nevada were once buried under broad alluvial plains, a result consistent with previous work. The reconstruction also supports the hypothesis that the gravels accumulated behind a high ridge along the Sierra Nevada foothills, with the Yuba River providing an important outlet. Moreover, gravel deposits on two high peaks indicate that the gravels may have buried the Feather River watershed up to the modern crest of the range. Finally, on the basis of our reconstruction, we estimate that the total volume of the gravels was, at a minimum, ~200 km³. The map that we created showing the original extent of the gravels was used as the cover image for the Dec. 2020 issue of AJS.

Minimal Net Incision of the Northern Sierra Nevada (CA) since the Eocene – Early Oligocene

Significant Late Cenozoic uplift (> 1000 m) of the northern half of the Sierra Nevada (CA), a mountain range in the North American Cordillera, has been a dominant paradigm over the past century. This paradigm has been supported by evidence suggesting that, in response to this recent uplift, the range’s deep canyons were incised in the past 3 – 4 Ma. However, paleochannel elevations compiled from a mining report and geological maps demonstrate that, while some modern rivers have incised 560 m below their Eocene – Early Oligocene riverbeds, incision by others has been less than 300 m. For example, Eocene – Early Oligocene fluvial gravels can be found just 161 m above the modern channel deep within the canyon of the South Fork American River. We conclude that the initation of Late Cenozoic incision was due to a resumption of a period of downcutting that was interrupted in the Eocene when the rivers were buried by fluvial sediment and by later volcanic deposits. This interpretation challenges the hypothesis that recent uplift was responsible for deep canyon incision. Correctly identifying the causes of recent incision in the northern Sierra Nevada has important implications for understanding the geological history of the North American Cordillera as the range is hypothesized to have been the western ramp of the Nevadaplano. This work was published in Geology; you can download the paper here (note, this is the proof).

Lithological and Structural Controls on River Profiles and Networks in the Northern Sierra Nevada (CA)

In this study, the strong lithological heterogeneity of the northern Sierra Nevada (CA) is exploited to elucidate the role of lithology on river profiles and patterns at the mountain-range scale. The analyses indicate that plutonic, metavolcanic, and quartzite bedrock generally host the steepest river reaches, whereas gentle reaches flow across non-quartzite metasedimentary rocks and fault zones. In addition, the largest immobile boulders are often in the steepest reaches, suggesting that wide joint spacing plays a role in creating steep channels, and a positive relationship between boulder size and hillslope angle highlights the coupling of the hillslope and fluvial systems. With respect to river network configurations, dendritic patterns dominate in the plutonic bedrock, with channels aligned down the slope of the range; in contrast, river reaches in the metamorphic belts are mainly longitudinal and parallel to the structural grain. These observations indicate that attempts to infer uplift and tilting of the range based on the gradients and orientations of paleochannel remnants should first account for the effect of bedrock erodibility. Indeed, the differences in gradients of Tertiary paleochannel remnants used to argue for late Cenozoic uplift of the range can be wholly explained by differences in lithology. This paper was published in Geological Society of America Bulletin, 132:655-667.

Profile of the North Fork Feather River showing the strong control of rock type on channel slope.

A Critique of the Evidence for Recent Uplift of the Sierra Nevada

From the earliest days of California geology, the ramp-like profile of the northern half of the Sierra Nevada mountains and putative signs of recent incision have been interpreted as evidence that the range was formed by the tilting of a rigid block in the late Cenozoic. Over the years, various geomorphic analyses have been used to quantify the magnitude of uplift and to establish its timing, such as analyzing the gradients of ancient channels, examining the tilt of sedimentary beds, and reconstructing the incisional history of rivers. Most studies that have used these methods concluded that there has been substantial (>500 m) recent uplift of the Sierra. In contrast, investigations based on other sources of paleotopographic information, such as isotope records, thermochronology, and detrital zircon geochronology have found that the Sierra have been at high elevations for much of the Cenozoic. This set of contradictory results motivated a re-examination of the geomorphic evidence for late Cenozoic uplift. I thoroughly examined these geomorphic studies and discovered that all but one suffered from flawed assumptions and that they were easily disproved by the geological evidence. For example, several studies based their results on reconstructions of ancient channels that would have flowed up and over bedrock ridges as high as 190 m, a physical impossibility. Furthermore, the conclusions from another widely cited study (Unruh, 1991) imply that the auriferous river gravels found along the South Yuba River were once 500 m below sea level - a fatally flawed result. Other weaknesses included unjustified assumptions regarding the original tilt of fluvial deposits, misinterpretations of stratigraphic relationships, and inadequate recognition of the effect of lithology on channel profiles. The studies supporting recent tilting in the northern Sierra Nevada, therefore, are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels used to argue for recent uplift is, instead, entirely consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. In addition, I demonstrate that deep northern Sierran canyons thought to have been recently incised were, in contrast, cut as early as the Eocene-Oligocene. However, two geomorphic studies from the southern Sierra are consistent with late Cenozoic tilting and uplift, although ongoing tectonic activity may be insignificant. Finally, I introduce a conceptual model of the evolution of the Sierran landscape, applicable primarily to the northern half of the range, illustrating the development of three different drainage networks since the late Jurassic (see figure below). This work was published in the American Journal of Science 314: 1224-1257 and cited in the popular press.

Geomorphic evolution of the northern Sierra Nevada.

(A) By the late Jurassic– early Cretaceous, a trellis network had developed, with a greenstone ridge forming a western drainage divide.

(B) In the late Cretaceous, headward erosion of rivers across the greenstone ridge begins to capture channels from the trellis network.

(C) Knickpoints moving up tributaries cut trenches through the longitudinal valleys. Hillslopes begin to shed their deep mantle of weathered regolith.

(D) By the Eocene-Oligocene, a transverse river system has developed. High sediment supply and a low transport capacity through the greenstone ridge leads to widespread aggradation and braided rivers; fine-grained material deposited west of the ridge creates the Ione Formation.

(E) Oligocene-Miocene volcanic eruptions bury much of the range.

(F) Rivers incise back down through the volcanic and fluvial sediment, eventually reoccupying the transverse network. Figures A–C modified from DeGraaff-Surpless and others (2002).

The Mystery of the Mima Mounds: SOLVED!

Mima mounds are small densely packed hills found on all continents except Antarctica. The most famous and well-studied ones are along the west coast of North America. Over the centuries, numerous hypotheses have been advanced to explain how they form, including periglacial processes, artesian pressure, gas venting and, of course, aliens. One hypothesis, proposed in the 1950s, was that gophers build them in response to seasonally flooded soils. To test this idea, I created digital gophers that burrow across a simulated landscape; the digging behavior of the digital gophers is based on data from a real Mima mound field. Although there is no explicit mound-building rule in the model, the digital gophers spontaneously begin to create Mima mounds. In addition, the spatial distribution of the mounds in the simulated landscape matches the distribution observed in real mound fields. This paper was published in Geomorphology and was featured by various news outlets including the BBC, The Economist, and the Huffington Post. Click on the video to watch the model in action.

How Is Bedrock Transformed into Soil?

Simply put, terrestrial life needs soil. Soil is a critical source of the nutrients and water that support life on land. Yet, the processes that turn rock into soil are not well understood. The biota seems to play an important part in breaking bedrock into smaller pieces and bringing them up closer to the surface. I have studied the role of tree roots in soil production through a numerical model that incorporates tree density, soil thickness, and rooting depth. The model predicts that soil is more quickly produced when there is already some soil on the surface. This work was published in the Journal of Geophysical Research 115, F04005, doi:10.1029/2009JF001526. Click on the video below to watch the model in action.

Wind Erosion after Fires in Southern California

It has been well documented that floods and debris flows often follow fires in the steep chaparral of southern California. Less appreciated is the role of fire in increasing the vulnerability of the soils to wind erosion. A satellite photo taken after a large fire in southern California shows what happens when these burnt soils are subjected to the strong Santa Ana winds that blow in the winter. A visit to one of these burnt sites provided evidence that the top 1-2 cm of soil had been stripped off by the wind on the ridges. To explore this poorly understood phenomenon, I began a suite of experiments that involved collecting intact samples of the soil surface, subjecting them to the range of temperatures measured in chaparral fires, and then placing them in a wind tunnel. The samples were taken from the San Ysidro range and were carefully removed from the soil using a 'cookie cutter' and placed in small cheesecake pans. Back in the lab (my backyard), the samples were heated, with a propane torch, according to a range of temperatures (up to 900 C) and durations (from 5 to 60 minutes). The burnt samples (still intact) were placed in a small wind tunnel capable of generating wind speeds of 22 m/s. The goal of the wind tunnel experiments were two-fold: (1) measure the critical shear velocity of the soil, and (2) measure the total amount of dust emitted. The results suggest that soil surface temperatures above 600C begin to destroy the fungal hyphae and polysaccharides that otherwise hold the soil together and prevent wind erosion. This project was funded by the NSF and published in Geomorphology, 217:182-192.

Erosion in the Himalayas

One day while having a beer with Dr. Doug Burbank (UC Santa Barbara), he asked me if I wanted to go to Nepal - I said "why not?"; this was my introduction to Himalaya-scale geomorphology. My part of the project (which turned into a post-doc position) was to analyze daily suspended sediment concentrations and chemical fluxes in rivers in the Annapurna region; this provided me with a nearly inexhaustable supply of data. I was also in charge of training local Nepali to record water levels and collect and filter water samples. A number of different studies have come out of this work.

One of the goals of the project was to determine the controls on the spatial distribution of erosion rates in the region. Precipitation is often invoked as a first-order control on erosion rates yet we have found that long-term erosion rates in the High Himalayas appear to be independent of climate such that erosion rates are spatially uniform despite an order of magnitude difference in precipitation. Modern-day erosion rates, however, do seem to be related to precipitation with rapid erosion on the wet and warm southern flank of the Annapurna region and slow erosion on the dry and cold northern region of the Himalayas and the Tibetan Plateau. We reconcile the spatially non-uniform modern rates with the spatially uniform long-term rates by suggesting that, in the north, glacial activity during the Ice Ages drives very rapid erosion that compensates for the slow erosion during the interglacials. This work was published in Earth and Planetary Science Letters 267: 482-494.

From the rainfall and suspended sediment data, I have identified rainfall thresholds that need to be overcome before landslides are triggered. Using a DEM of the field area, I have coupled a simple hillslope hydrology model with a slope stability analysis and have been able to reproduce the observed thresholds. This work was published in Geomorphology 63: 131-143.

Links between the shape of landforms and the climate are always assumed but unequivocal data supporting a connection have been difficult to find. Our field site in the Himalayas presents a great opportunity to look for a climatic signal in the shape of the mountains because of two key characteristics: 1) a steep precipitation gradient exists across the area, and 2) the hillslopes appear to be at their threshold angle, thus negating the influence of potential gradients in the uplift rate. An analysis of a 90m DEM, coupled with a 3-yr record of annual rainfall, suggests a relationship between mean slope angle and mean annual rainfall such that steeper slopes are found in the drier regions (see figure). This work was published in Geology 32: 629-632.

Chemical Weathering

Understanding the controls on chemical weathering of bedrock is key to closing the loop between climate, tectonics, and erosion. I have been analyzing chemical fluxes from 10 watersheds scattered across a steep climatic gradient in the High Himalayas to try to distinguish between climatic and topographic controls on the rates of chemical denudation. The results, published in Geomorphology 122: 205-210, indicate a strong positive linear relationship between erosion and chemical weathering.

A seminal paper by Raymo and Ruddiman suggested that the uplift of the Himalayas led to the drawdown of atmospheric CO2, thus triggering global cooling. They hypothesized that the exposure of fresh surfaces by rapid erosion accelerated the rates of chemical weathering and subsequent CO2 sequestration. This theory is based on the notion that increases in erosion rate are matched (or exceeded) by increases in weathering rates. To test this idea, I developed two numerical models that couple chemical weathering processes with erosional processes. They both indicate that, at the high end of erosion rates, increases in denudation are not matched by increases in chemical weathering. These papers were published in Earth and Planetary Science Letters 264: 259-265 and Geology 37: 151-154.

To examine the role of soil properties in controlling the rate of chemical weathering, I created a hillslope flume. The flume holds a slab of synthetic bedrock composed of gypsum and table salt. Materials with different hydraulic conductivities are then poured over the bedrock and water is introduced from the top of the flume to examine how seepage velocity controls chemical weathering rates. I found that the greatest weathering occurs in materials with low hydraulic conductivity but that, once the pore fluid reaches saturation, any further decreases in conductivity are irrelevant. The experiments also demonstrated that the least amount of weathering occurs when there is no soil at all, thus confirming one of GK Gilbert's intuitions. Finally, I derived a numerical model for predicting weathering rates as a function of hydraulic conductivity. This work was published in Geology 34(12): 1065-1068.

The Mobilization of Debris Flows from Shallow Landslides

The prevailing theory explaining how rigid landslides transform into debris flows assumes that soils are loose and collapse during failure; as the soil collapses, pore pressures shoot up and debris flow behavior initiates. Simon Mudd (University of Edinburgh) and I tested this theory by analyzing soils from areas that were the sources of debris flows and slumps (ie, failures that did not liquefy). Surprisingly, we found that all of the soils dilated during shear, even the ones that produced debris flows (see figure). This observation, coupled with the observations of others who have suggested that most natural soils are dilational, prompted us to re-examine the standard model of debris flow initiation via a numerical model. We concluded that a soil's potential for liquefaction is independent of porosity (i.e., how loose it is) but sensitive to sand content because of its effect on hydraulic conductivity. This paper was published in Geomorphology 74: 207-218.

Fire and Debris Flows

The relationship between fire and accerated erosion has been well-documented. I took advantage of a proscribed burn near my field site to do a detailed study of post-fire erosional processes. I found that a generally unrecognized process, thin debris flows, was responsible for the greatest amount of sediment transported off the burned hillslopes. These little debris flows begin as slope failures that are only 1-2 cm thick and occur right above a hydrophobic layer. I used the infinite-slope stability analysis to predict the amount of rain that it would take to trigger these after a fire and the results agree very well with observations made by others. This work was published in Earth Surface Processes and Landforms 28(12): 1341-1348.

With funding from the USDA, I investigated the role of ash in generating large progressively-bulked debris flows. These types of debris flows are not caused by an initial slope failure but appear to begin as clear overland flow that becomes a hyper-concentrated flow and then a debris flow. A paper describing the morphology of the gullies created by these types of debris flows was published in Geomorphology.

Soil Creep

With the goal of understanding how sediment is transported off of hillslopes, I've investigated a number of soil creep processes. These processes are generally characterized as being slope-dependent, meaning that the sediment flux is a function of slope. On the grassland hillslopes that I studied, the sediment flux by gopher bioturbation was the dominant process. While digging through the soil for plant roots, gophers move a considerable amount of soil. On the basis of simple measurements, I developed an empirically-derived sediment transport equation for gopher bioturbation. This work was published in Earth Surface Processes and Landforms 25: 1419-1428.

The main soil creep process on hillslopes in dry climates that have little ground-level vegetation is dry ravel and this process is particularly important immediately after fires. I derived a slope-dependent sediment transport equation, derived from the momentum equation, to describe this process. I tested the equation with a series of flume experiments and calibrated it with sediment traps set out in the field. This work was published in the Journal of Geophysical Research (PDF).

In the process of writing a review paper on the effects of bioturbation on soil processes, I derived slope-dependent sediment flux equations for tree-throw and root growth-and-decay. This paper was published in the Annual Review of Earth and Planetary Sciences (PDF).

Shallow Landslides

During the 1997-98 El Nino, record rainfall triggered over 150 shallow landslides with a 10 sq km area near Santa Barbara, California. I studied these failures to understand the mechanics of failure as well and to estimate the amount of sediment evacuated from the hillslopes. Additionally, many of the hillslopes at the site had been converted from coastal sage scrub to grasslands for grazing animals, so I also examined the effect of vegetation conversion on slope stability and long-term changes in soil depths. This work was published in GSA Bulletin 114: 983-990.

Overland Flow

Sediment transport by overland flow is often considered to be a transport-limited process where soil particles are just sitting on the soil surface waiting to be washed downslope. In soils that are high in sticky (i.e., smectitic) clays, the flow itself may be unable detach sediment particles. In this case, detachment of soil particles by raindrop impact provides the material that is transported by overland flow. I did a number of rainfall simulation experiments on a variety of hillslopes with various slope angles, vegetation cover densities, and rainfall intensities to understand the process of sediment detachment by drop impact. With the data from these experiments, I developed an expression, called rainpower, that is derived from a basic consideration of the kinetic energy of raindrops. This work was published in Water Resources Research (PDF).

I collaborated with Dr. Noah Fierer (now at CU Boulder) to investigate the transport of carbon and nitrogen, in both dissolved and particulate form, by overland flow. We specifically looked at the differences in nutrient transport between grassland hillslopes and sage-scrub hillslopes. We also did some trampling experiments with sawed-off cow hooves ("meat feet") to examine how hoof impact lowers the infiltration capacity of the soil. We found that conversion of sage-scrub to grasses for grazing animals increases the loss of carbon and nitrogen from the hillslopes. This work was published in the Journal of Environmental Quality (PDF).

Coupling the Hillslope and Fluvial Systems

The delivery of sediment to channels is an inherently stochastic process, largely dependent on the vagaries of climate. To explore how climate controls the magnitude and frequency sediment transport on hillslopes, I developed a computer model, driven by rainstorms and fires, that predicts the delivery of sediment. In the model, transport processes such as overland flow and shallow landsliding are explicitly represented. I used the model to examine the effects of climate-induced vegetation change on the amounts and tempo of sediment delivery in a steep, semi-arid landscape. The surprising conclusion from the model was that climatically-driven changes in vegetation, from a doubling of CO2, would have a greater effect on sediment delivery than just changes in the magnitude and frequency of climatic events. This work was published in Water Resources Research (PDF).

I collaborated with Dr. Noah Fierer and Dr. Oliver Chadwick (UC Santa Barbara) to adapt the sediment delivery model to predict the delivery of sediment-bound carbon, nitrogen, and phosphorus in a Mediterranean landscape. This involved detailed measurements of soil carbon, nitrogen, and phosphorus throughout our field site. The results from the model suggest that vegetation controls the tempo of nutrient delivery, which could have important implications for carbon sequestration and the eutrophication of water bodies. This work was published in JGR-Biogeosciences 10.1029/2005JG000032.

Tidal Channels

Despite highly sinuous planforms and evidence of active bank erosion, tidal channels have relatively slow rates of lateral migration. In an effort to solve this paradox, I closely monitored areas of deposition and erosion in a tidal channel in Marin County, California. This was done through the monthly measurement of 65 erosion pins placed along the length of the channel. Rates of erosion and deposition from the pin measurements were coupled with a bank stability analysis to develop a numerical model for predicting a rate of lateral migration that compared favorably to a rate estimated from a time series of aerial photographs. The paradox was solved through the realization that failed bank material persists for several years, thus protecting the bank from further erosion. This work was published in Estuaries 21(4B): 745-753.

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