Transverse and Longitudinal Variation in Woody Riparian Vegetation Along a Montane River

Abstract

This study explores how the relationship between flow and riparian vegetation varies along a montane river. We mapped occurrence of woody riparian plant communities along 58 km of the San Miguel River in southwestern Colorado. We determined the recurrence interval of inundation for each plant community by combining step-backwater hydraulic modeling at 4 representative reaches with Log-Pearson analysis of 4 stream gaging stations. Finally, we mapped bottomland surficial geology and used a Geographic Information System to overlay the coverages of geology and vegetation. Plant communities were distinctly arrayed along the hydrologic gradient. The Salix exigua Nuttall (sandbar willow) community occurred mostly on surfaces with a recurrence interval of inundation shorter than 2.2 years; the Betula occidentalis Hooker (river birch) community peaked on sites with recurrence intervals of inundation between 2.2 and 4.6 years. The hydrologic position occupied by communities dominated by Populus angustifolia James (narrowleaf cottonwood) was strongly related to age of trees and species composition of understory shrubs. The fraction of riparian vegetation on surfaces historically inundated by the river decreased in the upstream direction from almost 100% near Uravan to <50% along the South Fork of the San Miguel River. In upstream reaches much of the physical disturbance necessary to maintain riparian vegetation is provided by valley-side processes including debris flows, floods from minor tributaries, landslides, and beaver activity. Where valley-side processes are important, prediction of riparian vegetation change based on alterations of river flow will be incomplete.

Sand and sandbar willow: a feedback loop amplifies environmental sensitivity at the riparian interface

“…Sandbar willow is relatively Xood-tolerant and this would allow it to survive the common inundation at low elevation streamside zones (McBride and Strahan 1984;Krasny et al 1988a, b;Amlin and Rood 2001;Dixon et al 2002). Along the Salmon River it occurred in zones that are annually inundated and thus existed at the transition from the paraXuvial zone that is relatively barren of vegetation, and lowest Xood plain vegetation zone, which is less frequently inundated (Friedman et al 2006). In this transitional zone, the shrub would need to survive not only inundation that contributes to root anoxia, but also the erosive shear from swift water Xow (Darby 1999).…”

Terrain-based Predictive Modeling of Riparian Vegetation in a Northern Rocky Mountain Watershed

“…Site scale riparian studies have used field surveyed fluvially-associated terrain variables such as elevation above the channel, to account for the influence of flooding inundation and stream base flows on plant communities (Hupp and Osterkamp 1985;Auble et al 1994;Friedman et al 2006). The influence of hillslope (adjacent upland to stream) hydrologic inputs is related to the gradient of the site and the amount of area contributing water to that site.…”

“…The goal of our study was to use digitally-derived terrain variables and field-based vegetation data to create predictive models of the distribution of riparian vegetation across a 212 km 2 mountain watershed. The study approach was based on two primary assumptions grounded in well-documented terrain-hydrology and hydrology-vegetation relationships: (1) topographic position within valley bottom areas is a primary driver of hydrologic conditions including soil wetness, shallow groundwater level, and the magnitude and frequency of flooding inundation (Beven et al 1984;Rodhe and Seibert 1999;Seibert and McGlynn 2003) and (2) these ground and surface water sources in turn interact with environmental factors such as soil texture and geology to influence cross-valley vegetation gradients (Bendix and Hupp 2000;Castelli et al 2000;Auble et al 2005;Friedman et al 2006). Study objectives were: (1) to assess the strength of digitally-derived terrain variables for modeling the crossvalley extent of riparian vegetation, (2) to compare the strength of hillslope (adjacent upland to stream) versus fluvial (stream to adjacent upland) digital terrain variables within vegetation models, (3) to determine a threshold elevation above the channel to be used for coarse delineation of the riparian zone, and (4) to implement predictive models of riparian vegetation using digitally-derived terrain predictors in a geographic information system.…”

Quantifying flooding regime in floodplain forests to guide river restoration

“…While many studies have addressed site-specific factors favoring distinct floodplain vegetation (Connecticut River examples: Metzler and Damman, 1985;Nislow et al, 2002), few have attempted to translate this information into basin-scale predictions and decision-support (e.g. Friedman et al, 2006). For example, in many river systems, floodplain forests are more likely to develop in downstream locations, as a function of increasing basin size and decreasing stream gradient, which all serve to increase peak flow duration and extent, but these landscape determinants will manifest differently across ecoregional settings (Shankman and Hart, 2007).…”