Karen B. Gran

Riparian vegetation controls on braided stream dynamics

Riparian vegetation can significantly influence the morphology of a river, affecting channel geometry and flow dynamics. To examine the effects of riparian vegetation on gravel bed braided streams, we conducted a series of physical experiments at the St. Anthony Falls Laboratory with varying densities of bar and bank vegetation. Water discharge, sediment discharge, and grain size were held constant between runs. For each run, we allowed a braided system to develop, then seeded the flume with alfalfa (Medicago sativa), allowed the seeds to grow, and then continued the run. We collected data on water depth, surface velocity, and bed elevation throughout each run using image-based techniques designed to collect data over a large spatial area with minimal disturbance to the flow. Our results show that the influence of vegetation on overall river patterns varied systematically with the spatial density of plant stems. Vegetation reduced the number of active channels and increased bank stability, leading to lower lateral migration rates, narrower and deeper channels, and increased channel relief. These effects increased with vegetation density. Vegetation influenced flow dynamics, increasing the variance of flow direction in vegetated runs and increasing scour depths through strong downwelling where the flow collided with relatively resistant banks. This oblique bank collision also provides a new mechanism for producing secondary flows. We found it to be more important than the classical curvature-driven mechanism in vegetated runs.

Downstream variations in the width of bedrock channels

Field surveys of channel width w and drainage area A in bedrock channel reaches reveal relationships where w 5 cA b , similar to the classic hydraulic geometry of alluvial channels. Data from five mountain channel networks support the assumption used in many landscape evolution models that an alluvial hydraulic geometry relationship where b 5 0.3-0.5 holds for bedrock channel systems. Although there is substantial local variability in channel width in bedrock channel systems, there is no systematic difference in width versus drainage area relations for the surveyed bedrock and alluvial reaches in sedimentary lithologies in coastal Oregon and Washington. In contrast, bedrock channels were narrower, and therefore had deeper flow, than alluvial channels with equal drainage areas in the granite and limestone terrain of the Yuba River, California. In addition, data from the Mokelumne River show that bedrock channel width decreases substantially downstream at the contact between relatively weak limestone and more erosion-resistant granite, but that channel slope does not change appreciably across contacts between these two lithologies. Data from coastal Oregon drainage basins further show systematic channel widening after flood flows and debris flow impacts. We conclude that downstream variations in the width of bedrock channels generally follow traditional hydraulic geometry relations but also reflect the local influence of longitudinal patterns of bedrock erosivity and disturbance history.

Riparian Vegetation as a Primary Control on Channel Characteristics in Multi-Thread Rivers

Vegetation has been recognized as a primary control on river planform, particularly as a determinant of whether a river will adopt a braided or single-thread pattern (e.g. Millar [2000]). Studies have shown that overall behavior of the system correlates with vegetation type or density, shifting between a single-thread channel and a multi-thread system as vegetation changes [Mackin, 1956; Brice, 1964; Nevins, 1969; Goodwin, 1996; Ward and Tockner, 2000]. Murray and Paola [1994] concluded that braiding is the main mode of instability for unconstrained flow over a noncohesive bed. In other words, in the absence of cohesion to stabilize the banks and/or discourage formation of new channels, the flow tends to create new channels until a braided system develops. There-

Large Shift in Source of Fine Sediment in the Upper Mississippi River

Although sediment is a natural constituent of rivers, excess loading to rivers and streams is a leading cause of impairment and biodiversity loss. Remedial actions require identification of the sources and mechanisms of sediment supply. This task is complicated by the scale and complexity of large watersheds as well as changes in climate and land use that alter the drivers of sediment supply. Previous studies in Lake Pepin, a natural lake on the Mississippi River, indicate that sediment supply to the lake has increased 10-fold over the past 150 years. Herein we combine geochemical fingerprinting and a suite of geomorphic change detection techniques with a sediment mass balance for a tributary watershed to demonstrate that, although the sediment loading remains very large, the dominant source of sediment has shifted from agricultural soil erosion to accelerated erosion of stream banks and bluffs, driven by increased river discharge. Such hydrologic amplification of natural erosion processes calls for a new approach to watershed sediment modeling that explicitly accounts for channel and floodplain dynamics that amplify or dampen landscape processes. Further, this finding illustrates a new challenge in remediating nonpoint sediment pollution and indicates that management efforts must expand from soil erosion to factors contributing to increased water runoff.

Spatial and temporal patterns in fluvial recovery following volcanic eruptions: Channel response to basin-wide sediment loading at Mount Pinatubo, Philippines

The June 1991 eruption of Mount Pinatubo, Philippines, was one of the largest volcanic eruptions of the twentieth century, emplacing 5–6 km 3 of pyroclastic-fl ow material on the fl anks of the volcano. The combination of abundant, relatively fi ne-grained, easily erodible material and intense tropical rainfall led to numerous lahars immediately following the eruption. Even after major lahars ended, sediment yields in some basins remained orders of magnitude above pre-eruption levels. Using data collected from 1996 through 2003, we investigated fi ve basins that experienced varying amounts of sediment loading in the 1991 eruption, from 1% to 33% of the basin area covered by valley-fi lling pyroclasticfl ow deposits. From measurements of fl ow and bed characteristics made through time, we developed a general conceptual model for channel recovery following basin-wide sediment loading. Initially, fi ner-grained sediment and pumice are mobilized preferentially through selective transport. Once the bed is coarse enough for gravel-size clasts to interact with one another, clast structures develop, increasing form roughness and critical shear stress and inhibiting initial clast mobility. As sediment inputs continue to decline, the channel incises into valley bottom sediments, progressively armoring through winnowing. At Pinatubo, incision and armoring occur fi rst as dry season phenomena due to reduced sediment inputs, eventually moving to year-round low-fl ow bed stability. Observed timing of the onset of in-channel biological recovery suggests that reestablishment of channel stability helps catalyze aquatic ecosystem recovery.

Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.

Long-term elevated post-eruption sedimentation at Mount Pinatubo, Philippines

Post-eruption sedimentation rates from two basins draining Mount Pinatubo, Philippines, are compiled and extended through A.D. 2009 to compare short-term predictions with long-term yields. The Pasig-Potrero and Sacobia Rivers were covered by >1 km 3 of thick pyroclastic flow deposits during the 1991 eruption of Mount Pinatubo, blanketing 30% of each basin with highly erodible sediments. Erosion of these deposits led to some of the highest sediment yields ever recorded. Yields peaked in 1991 and declined exponentially for the first decade following the eruption. Deposition on the aggrading alluvial fan shows that sedimentation rates from 2001 to 2009 were almost twice as high as an exponential decay model would predict, with rates that have leveled off and are no longer declining exponentially. Patterns of long-term sedimentation are similar to those of the North Fork Toutle River (Washington State, United States) at Mount St. Helens, with two phases of erosion and sediment export. In the first, erosion of hillslope tephra and carving of the initial valley network lead to high sediment yields that decline exponentially over 5–10 yr. In the second phase, continued valley widening and fluvial instability maintain high sediment yields for at least two decades. A comparison of post-eruption deposition with pre-eruption yields indicates that century- to millennial-scale sedimentation is dominated by sediment associated with discrete eruptive events.

Landscape Evolution in South-Central Minnesota and the Role of Geomorphic History on Modern Erosional Processes

The Minnesota River Valley was carved during catastrophic drainage of glacial Lake Agassiz at the end of the late Pleistocene. The ensuing base-level drop on tributaries created knickpoints that excavated deep valleys as they migrated upstream. A sediment budget compiled in one of these tributaries, the Le Sueur River, shows that these deep valleys are now the primary source of sediment to the Minnesota River. To compare modern sediment loads with pre-European settlement erosion rates, we analyzed incision history using fluvial terrace ages to constrain a valley incision model. Results indicate that even though GSA Today, v. 21. no. 9, doi: 10.1130/G121A.1.

Lateral erosion in an experimental bedrock channel: The influence of bed roughness on erosion by bed load impacts

Physical experiments were conducted to evaluate the efficacy of bed load particle impacts as a mechanism of lateral bedrock erosion. In addition, we explored how changes in channel bed roughness, as would occur during development of an alluvial cover, influence rates of lateral erosion. Experimental channels were constructed to have erodible walls and a nonerodible bed using different mixtures of sand and cement. Bed roughness was varied along the length of the channel by embedding sediment particles of different size in the channel bed mixture. Lateral wall erosion from clear-water flow was negligible. Lateral erosion during periods in which bed load was supplied to the channel removed as much as 3% of the initial wetted cross-sectional area. The vertical distribution of erosion was limited to the base of the channel wall, producing channels with undercut banks. The addition of roughness elements to an otherwise smooth bed caused rates of lateral erosion to increase by as much as a factor of 7 during periods of bed load supply. However, a minimum roughness element diameter of approximately half the median bed load particle diameter was required before a substantial increase in erosion was observed. Beyond this minimum threshold size, further increases in the relative size of roughness elements did not substantially change the rate of wall erosion despite changes in total boundary shear stress. The deflection of saltating bed load particles into the channel wall by fixed roughness elements is hypothesized to be the driver of the observed increase in lateral erosion rates.

Channel Morphology Response to Selective Wood Removals in a Sand-Laden Wisconsin Trout Stream

Abstract Large sand bed loads in trout stream headwaters can limit salmonid spawning habitat and reproductive success. This phenomenon has been observed in many northern Wisconsin watersheds, where historic logging practices are the likely source of the sediment loading. Presently, sediment transport is limited by abundant woody debris, causing channels to aggrade and bury gravels. We evaluated the impacts of a wood debris and beaver dam removal remediation strategy on fine sediment transport and exposure of the underlying gravel and cobble substrates in a second-order Lake Superior tributary. A 300-m treatment reach received selective wood removals and was compared with both an upstream 300-m reference reach receiving no alteration and a downstream reach to monitor the effects of transferred fine sediment. Physical channel measurements were taken before the wood removal process, with repeat sampling at 10 and 12 months posttreatment. The wood removal treatment resulted in a significant 25% narrowing of m...