Impact of spring-associated riparian vegetation on channel morphology : insights from Henry Mountains, UT




Southard, Paul Johnson

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Riparian vegetation is a key control on alluvial channel morphology, but a more quantitative understanding of its morphological impacts must be developed in order to predict channel change in the face of riparian succession and prescribe vegetation as a river management tool. Recent studies have focused on vegetation's role as a bank-strengthening agent that hinders braiding, increases depth and decreases width in perennial streams. However, the tendency of vegetation to grow on the channel bed, in addition to the banks, in ephemeral streams suggests that it may have entirely different morphological impacts. I examined dryland channels draining the Henry Mountains in southern Utah that are intersected by perennial bedrock springs along their length. Spatial variability in water availability from the spring produces high spatial variability in vegetation. As a result, these channels provide a unique opportunity to study how channel morphology responds to changes in vegetation, and to identify the morphological impact of riparian vegetation in channels that accommodate ephemeral streams. I used 1m-resolution Digital Elevation Models and AnuGA flow modeling software to simulate five discharge scenarios for two roughness conditions in three of these channels, and produced high-resolution datasets of channel geometry and spatial vegetation density. I tested the hypotheses that 1) riparian vegetation has a quantifiable impact on channel morphology in ephemeral streams and 2) the morphological impact of riparian vegetation in channels with channel bed vegetation is different from that in channels with channel bank vegetation. I quantitatively showed that, for these ephemeral streams, channel morphology is dependent on vegetation characteristics. In channels with channel bed vegetation, vegetation causes widening and decreases flow velocity. In channels with only channel bank vegetation, vegetation causes narrowing and deepening. By performing comparisons of sparsely- and densely-vegetated reaches within individual channels, I provided the following quantitative constraints on the relative magnitude of riparian vegetation's impact on channel geometry: Reaches with dense channel bank vegetation are approximately 25% narrower and 25% deeper than sparsely-vegetated reaches. Channel bank vegetation causes narrowing by 12-16% and deepening by 7-11% per 0.1 increase in calculated LiDAR Vegetation Index from the unvegetated cross-section channel geometry. Reaches with dense channel bed vegetation are approximately 20-100% wider and 5-30% slower than sparsely-vegetated reaches. Channel bed vegetation causes widening by 13-29% and slowing by 2-7% per 0.1 increase in calculated LiDAR Vegetation Index from the unvegetated cross-section channel geometry.


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