The Effects of Variable Precipitation on Discharge and Sediment Transport in Streams in the Teton Mountain Range, Wyoming, USA
Abstract
Discharge and sediment distributions control the efficiency of sediment transport and incision into bedrock units in active stream channels. The efficiency of stream erosion is an important factor influencing the evolution of mountain landscapes. Variations in yea rly precipitation affect the timing of snowmelt, and therefore, the water availability for discharge in high elevation streams. This study explores how differences in annual precipitation can impact alpine stream erosion. Water discharge, bed load sediment s, and suspended solids were observed for major streams draining watershed areas between 10 km² and 43 km² in the Teton Mountain Range in northwestern Wyoming, USA. The maximum sediment sizes capable of being moved through the stream channels at late summer flow conditions were determined using basal shear stress and critical shear stress calculations. Annual precipitation data over 2 years was compared with sediment transport conditions to compare how precipitation impacted erosion. Erosion proved to be effective in both high and low precipitation conditions; however higher precipitation resulted in prolonged snowmelt, higher discharge, greater sediment transport, and therefore higher erosional efficiency.
Keywords
Download Options
Introduction
Streams play an important role in the evolution of many mountain landscapes by acting as transport mechanisms to move sediments from high to low elevations (Whipple et al., 2000; Kirby and Whipple, 2001; Tomkin et al., 2003).River incision into bedrock is a key erosion process controlling the rate of landscape responses to change in rock uplift rate and climate in mountainous areas (Howard, 1998; Whipple et al, 2000).The efficiency of stream erosion is influenced by the availability of water and sediments in the channel, which provide energy and tools respectively to drive transportation and incision. In alpine systems, water sources include precipitation, snowmelt and glacier melt, all of which are sensitive to small changes in climate conditions (Wendel, 2015). Stream sediments are sourced from the active channel, colluvial deposits below steep hillslopes, and glacial moraines or till (Wohl, 2005).
Precipitation is an important factor influencing stream discharge and erosion. Rainfall runs overland to enter the stream channel; and snowmelt accumulates over winter and melts throughout the summer at high elevations in alpine mountains. Intense storms or extreme temperatures driving rapid snowmelt can cause downstream flooding. When streams reach high flow or flood-like conditions, channel morphology undergoes its greatest changes (Leopold and Maddock, 1953). At high discharge, streams are more likely to incise bedrock, creating deeper flow and steeper slopes (Park 1977).
Sediment sizes and volumes carried by streams are also indicators of how much erosion occurs within watersheds (Tomkin et al., 2003). Sediments carried by streams act as tools to abrade streambeds and cause incision (Sklar and Dietrich, 2001). If sediments accumulate in a thick layer in the streambed, erosion is focused further upstream (Wohl, 1998). As rock erodes from its source and is carried downstream, 30% of sediment eroded will be transported through the length of the stream system to the mouth (Walling, 1983). Maximum erosion occurs when bedrock is only partially exposed under a coarsegrained supply of sediment. Fine-grained sediments in streams (clay, silt, and sand sized particles) abrade channels less effectively because they are mostly transported in suspension (Sklar and Dietrich, 2001). Larger clasts, including gravel, pebbles, cobbles, and boulders, therefore, play a more important role in stream abrasion and incision.
Small mountain streams receive comparatively less attention than larger, alluvial rivers due to difficulties in accessing and monitoring these systems (Montgomery and Gran, 2001). To aid in understanding how small stream systems effectively erode the landscape, we investigate summer stream flow in the Teton Range in northwestern Wyoming over two years with different precipitation records. The Teton Mountains have a distinct landscape influenced by glacial, fluvial, and mass wasting erosional forces. The result of these forces is a steeply sloping topography lacking vegetation on the mountain slopes composed of resistant bedrock. Streams were previously studied to understand the rates and patterns of erosion in Garnet and Cascade Canyons (Tranel et al., 2011 and 2015). Mass wasting provided many sediments from high elevation hillslopes; and glacial erosion caused limits to erosion (Tranel et al., 2011). Although the previous studies used stream sediments, they revealed more information about the importance of hillslope and glacial erosion than the efficiency of modern streams in the Teton Range.
In this project, we assess how efficiently the streams erode their respective canyons. To do this, we measured stream discharges in different watersheds in the Teton Range to examine sediment transport, and how it is controlled by yearly variations in precipitation. Bed load sediment sizes and suspended solids were analyzed in four watersheds of the Teton Range to determine how catchments vary in the materials they transport. By researching how snowmelt influences sediment transport in the Teton Range, we hope to better understand how the stream system contributes to the evolution of this complex landscape.
Conclusion
The results of our study begin to capture a record of stream discharge for channels draining the eastern flank of the Teton Range. Our findings also help us understand the potential for erosion in these mountain streams. We observed discharge and sediments during high flows in 2011 related to snowmelt, however they were not flood conditions. Late summer snowmelt directly influences stream discharge in mountain streams, which, in turn, allows for efficient sediment transport. Landscape features, including relief and elevation, may influence discharge and erosion potential more than catchment area in the Teton Mountains because snowmelt from high elevation can be prolonged during warmer years with lower precipitation.
This work begins an assessment of flow conditions in the Teton Range; however there remains much to be done to understand these mountain stream systems. More intensive monitoring throughout the year for several years would provide a more precise evaluation of the changes related to seasonal changes and individual precipitation events. Continuous monitoring or consistent monitoring at the same time of day will prevent discrepancies in data related to fluctuations in discharge caused by changes in daily temperatures. We also see interesting trends possibly related to elevation and catchment area, therefore a survey of many more streams in the range would help assess how the transport and erosion dynamics may be different related to size and relief. Lastly this study could be improved by more intensive or accessible sediment sampling strategies. In the faster 2011 flows, we observed smaller maximum grain sizes than in the slower 2012 flows. One possibility for this unexpected sediment size observation could be limitations to collecting in rapid and high flow (Wolman, 1954). When discharge is as high as we observed, it is possible that we were not able to collect in the fastest flowing area of the stream in 2011. If we were to use a sediment collector, we may be able to more actively characterize the sediments across the entire channel and capture all representative sediment size distributions.
Continued efforts to monitor discharge from mountain streams are important to understand the importance of snowmelt discharge in transporting sediments and supplying water resources as climate changes. Climate models indicate that snowpack in mountains in the western United States will change with a warming climate (Scalzitti et al., 2016). Those changes will thereby influence the timing of snowmelt discharge. Cooler temperatures tend to produce lower snowmelt discharges throughout the warm seasons as we saw in 2011, but overall warmer temperatures could result in continuous discharge as snowmelt throughout longer periods of the year, if any of the precipitation falls as snow at all (Molini et al., 2011). Snowmelt discharges from the Teton Mountains flow into the Snake River, which provides water to support agricultural irrigation and livestock (Clark et al., 1998). Variability in the timing of snowmelt discharge influences water availability for crops at different stages of the growing season, therefore it is important to understand the current dynamics of mountain streams because these systems are likely to change with changing climate in the future.