Aggrading river systems create their own stratigraphy, which can be very heterogeneous in the cross-stream, downstream, and vertical directions. If conditions change such that a river becomes degradational and cuts through its self-formed stratigraphy, the potentially abrupt grain-size transitions associated with this exhumation may lead to feedbacks on morphodynamic evolution that are difficult to predict a priori. We are exploring these surface-subsurface interactions during channel aggradation and degradation, using flume experiments to guide the development of a 2D numerical model. We expect this work to improve our ability to model the dynamics of rivers and their stratigraphy, and ultimately to improve our geologic interpretation of the stratigraphic record.

Whitewater parks are popular recreational amenities, especially in Colorado. These in-stream hydraulic structures typically create waves by constricting flow through a chute to increase velocities and form a hydraulic jump. These hydraulic conditions have been shown to potentially inhibit upstream fish movement. We are using 2D and 3D hydraulic models to compute a continuous and spatially explicit description of velocity and depth along potential fish swimming paths in the flow field, and the ensemble of potential paths are compared to fish swimming performance data to predict fish passage via logistic regression analysis.

Wildfires can dramatically change the hydrologic response of a watershed, leading to more overland flow and increased erosion rates for a given storm. Fires can therefore cause changes to hillslope and channel morphology, cause increased delivery of sediment to channels, impact water quality and aquatic habitat, and potentially influence landscape evolution. In the summer of 2012, the High Park fire burned 87000 acres of forest near Fort Collins, Colorado. We are using a combination of remotely-sensed topography, field observations of hillslope-scale sediment production, and Total Station, GPS, and terrestrial LiDAR surveys of channel geometry and channel network structure to better understand and quantify post-fire erosion rates at the watershed scale, sediment storage on hillslopes and sediment delivery to channels, changes in peak flows and channel morphology, and expansion and contraction of the channel network.

Run-of-river (RoR) hydropower projects have relatively small head ponds formed by small dams. The geomorphic impacts of these projects are thought to be small compared with large reservoir-based projects owing to their size and design, but the transient geomorphic response due to temporary sediment supply disruption is not well understood. We have been using morphodynamic models to explore potential downstream geomorphic changes due to installation of RoR projects so that we may better understand conditions that pose the greatest risk to aquatic habitat.

Structure-from-motion (SfM) photogrammetry has become widely used for topographic data collection in field and laboratory studies. We are using SfM extensively in laboratory experiments, and have conducted a rigorous analysis of SfM performance in lab settings. Our results show that SfM provides topographic data of similar accuracy to terrestrial laser scanning (TLS), at higher resolution and lower cost. We have suggested protocols for image acquisition and SfM software settings to achieve best results. In general, convergent imagery, taken from a higher angle, with at least several overlapping images for each desired point in the flume will result in an acceptable point cloud potentially capable of discriminating individual gravel particles.

Bedrock rivers differ from their fully alluvial counterparts in that they are able to transport more sediment than what is supplied from hillslopes and upstream sources. We have been advancing our understanding of the morphodynamics of mixed bedrock-alluvial rivers by developing analytical and numerical models of flow, sediment transport, sediment cover, and bedrock erosion. These models have been used to explore how bedrock channel cross-sectional shape evolves in response to impacts from sediment particles eroding the underlying bedrock, how bars form in mixed bedrock-alluvial river bends, and how sediment cover is distributed across the channel bed.

The ubiquitous effects of land use change on stream hydrologic and geomorphic processes present an ongoing challenge for hydrologic design at the interface of built and natural environments. The purpose of this project was to develop a scientifically supported method for defining the design hydrology that may be applied to stream crossings or other channel modification/restoration efforts, along with an understanding of how that design hydrology might change with land use changes.

The beds of nearly all gravel-bed rivers, and many gravel-bed flumes, are frequently arranged into patches of distinct grain size and sorting. Some of these patches have been observed to persist over timescales of decades, while others are able to freely move downstream. Bed surface heterogeneity may also be a primary control on sediment transport rates, and because bed material exerts a strong control on the near bed hydraulic environment, patches may have important implications for aquatic ecology. Flume experiments my colleagues and I have conducted have provided insight on how sediment supply affects the development of freely mobile patches, and how temporally-stable fixed patches are a consequence of interactions between the fluid flow field, the sediment transport field, and bed topography. In collaboration with the USGS Geomorphology and Sediment Transport Laboratory, I have developed a two-dimensional morphodynamic model simulating the mixed-grain-size sediment transport and bed evolution and used it to show how patches interact with the evolving bed, and, through their effect on the flow field, considerably affect bed morphology. I also have drawn on work in the fields of graph theory and computer vision to explore how patches can be delineated from high-resolution grain size datasets.

Tidal channels are fundamental features of estuarine and lagoon environments. These channels often exhibit alternate bar morphology, which can have a significant impact on channel navigability and the distribution of ecological habitat. Recent theoretical and experimental work has raised some fundamental questions regarding the evolution of tidal channels, such as: To what extent does the morphology of tidal channels reflect their evolutionary history? Does the equilibrium condition of tidal bars depend upon the initial channel configuration? Is bar instability in tidal channels absolute or convective? And, ultimately, to what extent are bar characteristics actually predictable? My colleagues at the University of Genoa and I are exploring these questions through the development of a two-dimensional morphodynamic model that simulates bar formation and evolution in tidal channels.

Urbanization can have a dramatic effect on a watershed. The development of roads, buildings, and other impervious areas tends to increase runoff ratios and produce very rapid hydrologic response times. Much of the drainage network can be man-made, in the form of culverts and storm drains, and stormwater detention basins can change the timing and magnitude of flood peaks by delaying the transport of runoff into streams. Floods in urban environments are very real hazards as typical thunderstorms can produce extraordinarily high discharges. Urban stream channels then have to somehow cope with this flashy hydrologic regime, and often become ‘unstable’ by eroding or incising excessively. My collaborators at Princeton and UMBC and I have been using Baltimore County, MD, as a field site to study various aspects of urban hydrology and flood hydraulics. Our work is part of the Baltimore Ecosystem Study (BES), a NSF Long-Term Ecological Research site (LTER). Additionally, colleagues at CSU have been investigating how urbanization affects the entire distribution of flows (i.e., the flow duration curve), using several watersheds in the Puget Sound region as a test case.

When springs occur in topographically flat, arid environments, they tend to form mound-shaped deposits through mineral precipitation processes. These so-called ‘mound springs’ occur relatively infrequently on Earth, but where they do occur they are generally important sources of water and can become focal points for unique ecological systems. The range of observed spring deposit morphologies suggest that mound springs follow a common evolution. Following insights gained from field expeditions to mound spring clusters in central Australia and in New Mexico, Michael Manga, Mary Bourke, Jon Clarke, and I have developed a model for the evolution of these deposits that relates mound shape to local subsurface hydrologic conditions. Because these mounds are essentially permanent features on the landscape, our modeling should allow us to use the mound shape to make inferences on historical local hydrogeology and paleoclimate. This may be especially useful on Mars, where high-resolution imagery of the planet’s surface has revealed features that are strikingly similar to terrestrial mound spring deposits. By applying our model to these features, we may be able to learn something about the hydrogeologic conditions that once existed in the martian subsurface.