Glen Leverich - Academia.edu (original) (raw)

Papers by Glen Leverich

Fen-wetland ecosystems are rare nationwide. Their unique groundwater regime and chemistry, along ... more Fen-wetland ecosystems are rare nationwide. Their unique groundwater regime and chemistry, along with a floating, vegetated peat mat that may occur, support diverse and rare plant and wildlife communities. A fen’s ecological benefits are considered even greater within an urbanized setting through its natural attenuation of runoff and pollutants, though these ecosystem services may diminish should the urban-sourced impacts eventually alter the fen’s bio-physical condition. Here we will present how determining a fen’s key bio-physical factors and understanding their combined sensitivity to external processes is necessary to define and address potential threats to a fen’s conservation. Located within a 100-acre Metro-owned natural area along the lower Willamette River, the last known remaining fen composed of a groundwater-fed lake with a densely vegetated floating peat mat in the region is vulnerable to threats that could alter its fragile biochemistry. Threats include stormwater runo...

The data release presents observations of riparian vegetation, topography, sediment quality, and ... more The data release presents observations of riparian vegetation, topography, sediment quality, and river corridor geomorphology in four river reaches of the Lower Virgin River extending downstream 62 river kilometers (rkm) from near the town of Littlefield (AZ) and the Arizona-Nevada border at rkm 119. Methods included field observations and analysis of remotely-sensed data before (2010) and after (2011-2012) a 40-year return period flood (December 2010, at the gaging station 'Virgin River near Littlefield'; (USGS gage #09415000)). The data release includes four .csv files related to field observations: UTM coordinates of field transect locations; vegetation and geomorphology; species codes; and sediment quality. The data release also includes three shapefiles describing the river corridor geomorphology obtained through the interpretation of airborne color orthophotography: before the flood; after the flood; and the union of these two. Finally, there are four metadata (.xml) f...

Ecohydrology, 2019

Short-term geomorphological and riparian vegetation responses to a 40-year flood on a braided, dr... more Short-term geomorphological and riparian vegetation responses to a 40-year flood on a braided, dryland river

Earth Surface Processes and Landforms, 2018

This research was facilitated by grants to the Marin County Department of Public Works, San Franc... more This research was facilitated by grants to the Marin County Department of Public Works, San Francisco Estuary Project and the Association of Bay Area Governments. We are indebted to Bill Dietrich for critical reviews of reports produced as part of this study, and to Derek Booth, Neil Roberts, Sara Rathburn and three anonymous referees for critical reviews of this manuscript. We are very grateful to Marin Municipal Water District, Marin County Open Space District,

This technical report summarizes the methods and results of a comprehensive riparian restoration ... more This technical report summarizes the methods and results of a comprehensive riparian restoration planning effort for the Gila Valley Restoration Planning Area, an approximately 53-mile portion of the upper Gila River in Arizona. This planning effort has developed a Restoration Framework intended to deliver science-based guidance on suitable riparian restoration actions within the ecologically sensitive river corridor. The framework development was conducted by a restoration science team, led by Stillwater Sciences with contributions from researchers at the Desert Botanical Garden (DBG), Northern Arizona University (NAU), University of California at Santa Barbara (UCSB), and U.S. Geological Survey (USGS). All work was coordinated by the Gila Watershed Partnership of Arizona (GWP), whose broader Upper Gila River Project Area is depicted in Figure 1-1, with funding from the Walton Family Foundation’s Freshwater Initiative Program.

Proceedings of the National Academy of Sciences, 2009

Meandering rivers are common on Earth and other planetary surfaces, yet the conditions necessary ... more Meandering rivers are common on Earth and other planetary surfaces, yet the conditions necessary to maintain meandering channels are unclear. As a consequence, self-maintaining meandering channels with cutoffs have not been reproduced in the laboratory. Such experimental channels are needed to explore mechanisms controlling migration rate, sinuosity, floodplain formation, and planform morphodynamics and to test theories for wavelength and bend propagation. Here we report an experiment in which meandering with near-constant width was maintained during repeated cutoff and regeneration of meander bends. We found that elevated bank strength (provided by alfalfa sprouts) relative to the cohesionless bed material and the blocking of troughs (chutes) in the lee of point bars via suspended sediment deposition were the necessary ingredients to successful meandering. Varying flood discharge was not necessary. Scaling analysis shows that the experimental meander migration was fast compared to ...

Environmental Management, 2014

We present here a method to integrate geologic, topographic, and land-cover data in a geographic ... more We present here a method to integrate geologic, topographic, and land-cover data in a geographic information system to provide a fine-scale, spatially explicit prediction of sediment yield to support management applications. The method is fundamentally qualitative but can be quantified using preexisting sediment-yield data, where available, to verify predictions using other independent data sets. In the 674-km 2 Sespe Creek watershed of southern California, 30 unique ''geomorphic landscape units'' (GLUs, defined by relatively homogenous areas of geology, hillslope gradient, and land cover) provide a framework for discriminating relative rates of sediment yield across this landscape. Field observations define three broad groupings of GLUs that are well-associated with types, relative magnitudes, and rates of erosion processes. These relative rates were then quantified using sedimentremoval data from nearby debris basins, which allow relatively low-precision but robust calculations of both local and whole-watershed sediment yields, based on the key assumption that minimal sediment storage throughout most of the watershed supports near-equivalency of long-term rates of hillslope sediment production and watershed sediment yield. The accuracy of these calculations can be independently assessed using geologically inferred uplift rates and integrated suspended sediment measurements from mainstem Sespe Creek, which indicate watershed-averaged erosion rates between about 0.6-1.0 mm year-1 and corresponding sediment yields of about 2 9 10 3 t km-2 year-1. A spatially explicit representation of sediment production is particularly useful in a region where wildfires, rapid urban development, and the downstream delivery of upstream sediment loads are critical drivers of both geomorphic processes and land-use management. Keywords Sediment yield Á Wildfire Á Urban development Á GIS Á Geomorphic landscape units Á California

The development of a mechanistic understanding of channel geometry and morphodynamics has been in... more The development of a mechanistic understanding of channel geometry and morphodynamics has been inhibited by the inability to create self-formed, freely meandering, single thread channels in a laboratory flume. By being able to reliably generate such channels, studies of the influence of sediment supply and flow dynamics as well as bank strength on channel morphology can be experimentally explored. We have found that the key experimental controls are: 1) ratio of bank strength to boundary shear stress exerted on the bank; 2) bedload and suspended load rates; and 3) variable flow discharge. We have been able to create meandering channels in a sand bedded laboratory flume using alfalfa sprouts. The alfalfa sprouts decrease the bank erosion rate so that bank erosion would occur at approximately the same pace as bar growth. The addition of coarse suspended load was necessary to cause deposition on bars to grow to the floodplain height. The sprouts contributed to deposition by creating a rough floodplain surface. Steady discharge failed to produced meandering, apparently due to the lack of suspended load deposition on the bar surface. The channels were created in a 3.6-m wide and 6.1-m long flume with an adjustable slope set at 0.01. We introduced both bedload (sand) and suspended load (crushed silica) into the top of the flume, which has an initial channel with either one or two bends carved into the floodplain. Runs lasted between 1 and 4 hours and occurred once per week. Alfalfa seeds were spread evenly outside the low flow channel following each run and are allowed to grow between runs. With the same material and flow conditions, the channel rapidly braided without the alfalfa sprouts. Braided was also favored under steady flow conditions. Under dynamic flows with banks strengthened by sprouts, the resulting experimental channels had many of the features observed in meandering streams such as oxbow lakes and meander cutoffs. The cutoffs occurred during overbank flows when high flow channels were reoccupied. As the portion of the flow passing through the reoccupied channel increased, an upstream-propagating headcut was initiated. Once the headcut propagated past the upstream junction with the main channel, sediment deposition blocked the upstream end of a secondary channel. The cutoffs became oxbow lakes when rapid bar growth promoted lateral channel migration away from the downstream junction with the cutoff channel. With these results in hand we are completing the construction of a larger flume in which we will set forth experiments on the influence of sediment supply, discharge magnitude and duration, grain size and bank strength on channel geometry.

Current efforts to restore floodplain river channels are limited by our poor understanding of the... more Current efforts to restore floodplain river channels are limited by our poor understanding of the controls on channel width and other morphologic attributes of meandering floodplain rivers. Restoration projects are commonly implemented downstream of dams where the supply of both water and sediment has been drastically altered. An emerging restoration strategy involves down-sizing the channel to restore a suite of active geomorphic processes including lateral migration of the channel and floodplain construction by bar growth and overbank deposition. Common channel design methods emphasize selection of the appropriate bank-full discharge and rarely consider the role of higher magnitude flows in shaping the channel and influencing rates of morphologic change. Here we report preliminary results of physical modeling experiments in which we have created laterally migrating stable-width channels in a laboratory floodplain basin. In these ongoing experiments we are exploring the influence of variable discharges on channel geometry and rates of planform evolution. In particular, we focus here on the use of hydrographs with peaks that result in over-bank flooding and sediment deposition. We use alfalfa and a mix of sand and silt to develop bank cohesion and assist bar growth while simultaneously varying discharge following predetermined hydrographs. The experiments are carried out in a 6.1 m long by 3.6 m wide flume with an adjustable slope set to one percent. A channel with constant width and sinuosity is initially carved into the floodplain prior to an experimental set of runs. We feed sand and silica silt, which move as bedload and suspended load, respectively, at the channel entrance. We use a point gage to measure flow depth and water surface slope, document planform evolution using overhead digital photos taken every minute, and create topographic maps with a line-laser photogrammetry technique. Preliminary results are consistent with our hypothesis that a variation in flow is required to maintain a stable width channel by growing bars up to floodplain height, stabilizing banks by depositing fines upon the floodplain and growing levees. Steady flow leads to channel widening and eventual braiding due to bar disconnection with the floodplain and thus creates a positive feedback cycle of runaway deposition and downstream sediment starvation. This phenomenon can be suppressed by a variation in flow magnitude, provided that bars have connected fully to the floodplain and well-developed levees are able to form, which help to stabilize channel banks. We will continue these experiments in the existing model floodplain and in a newly-constructed larger basin where the effects of entrance and exit conditions can be minimized.

Large water storage dams have altered the morphology of many rivers by trapping sediment and decr... more Large water storage dams have altered the morphology of many rivers by trapping sediment and decreasing the magnitude and duration of the seasonal peak flows that had been responsible for channel forming processes involving sediment transport. We hypothesized that a full range of flows, characteristic of pre-dam conditions, are critical for developing a dynamic, yet sustainable meandering floodplain river. We explored the role of variable flow as represented by a typical asymmetrically-shaped flood hydrograph in our physical model experiments of a scaled-down, gravel-bedded, meandering floodplain channel. Our model channel had erodible bed and banks and was allowed to evolve within a wide floodplain having a slope of one-half of a percent. Bank strength was provided using alfalfa sprouts and a thin layer of uniformly applied silica on the coarse floodplain sediments. We used a stepped hydrograph with a peak that resulted in overbank flooding, increased lateral migration rates, bar-connection with the floodplain, and stable channel width. The highest rates of bank erosion, bar growth, and lateral migration occurred during the overbank flows. We co-varied the bedload feed rate with the discharge while the sediment flux remained closely balanced over the course of each of the hydrograph runs, which was chiefly due to their shape. This occurred as the relatively longer durations of the final two hydrograph steps, where output rates exceeded input rates, compensated for the net increase of sediment storage within the channel during the rising limb steps. We additionally discovered that the ratio of deposition rates to erosion rates were not a function of discharge, but rather were dependent on the channel position relative to the apex of the migrating bend. Our research marks the first attempts at quantifying a channel's response to a systematic variation of flow in a three-dimensional physical model of a meandering floodplain river. This work applies directly to influencing restoration design methods that, in the past, have rarely considered the role of higher magnitude flows in shaping the channel and influencing rates of morphologic change.

As channels migrate across their floodplains they erode their outer banks and create new floodpla... more As channels migrate across their floodplains they erode their outer banks and create new floodplain on their inner bank via deposition of a point bar. While there has been considerable work on flow through bends and on erosion of the outer bank, there has been comparatively little study of the mechanisms of lateral point bar growth and accretion. We hypothesize

Stream restoration projects often involve constructing single-thread meandering channels, and the... more Stream restoration projects often involve constructing single-thread meandering channels, and their success requires understanding the conditions necessary to develop and maintain a meandering pattern. Empirical studies indicate that meandering channels occur under a specific range of width-depth ratio, slope and Froude number. We hypothesize, however, that meandering rivers also require: 1) bank strength from either cohesive material or vegetation, 2)

Ecohydrology, 2019

Earth Surface Processes and Landforms, 2018

Proceedings of the National Academy of Sciences, 2009

Environmental Management, 2014