The mapping of valley bottoms provides an initial assessment of the extent and distribution of potential riparian areas within a study site. A variety of semi-automated methods have been developed to map valley bottoms using geomorphic models. Each method is slightly different, but all of them rely on an analysis of the topography and strive to differentiate the valley bottom from the surrounding valley slopes and ridge tops. It is important to remember that these methods are attempting to automate a process that can be difficult, even for an analyst, to perform manually. The results can vary substantially across a study area and are dependent on the quality of the input data (e.g., DEM). Each mapping effort should be carefully reviewed to assess the acceptability of the product.
Click on each link below to learn more about each method.
- Valley Bottom Tool – a relatively simple tool based on hydrologic modeling and buffers.
- RSAC Riparian Mapping Tool – a sophisticated tool that relies on logistic regression modeling to identify valley bottoms.
- HGVC Tool – a sophisticated tool developed by Colorado State University that incorporates hydrologic parameters such as 100-year flood stage.
- Land Facet Corridor Tool – an easy and reasonably effective tool based on relative slope analysis.
All of these methods utilize digital elevation models (DEMs) for geomorphic modeling. DEMs typically have 30 m or 10 m resolutions; however, higher-resolution DEMs can be obtained from lidar data. Lidar-derived DEMs can improve results, but they also introduce new difficulties. All of the methods presented here were designed for 30 m and 10 m DEMs.
Buffers are an alternative to geomorphic models that have long been used to protect water quality. Buffers are routinely used in a variety of GIS applications to identify the area within a given distance of an object. In this instance, a buffer can be applied to a streamline (e.g., USGS hydrography data) to identify the total area within an established distance of that stream. Buffers are fast, easy, and inexpensive; however, they can only represent a crude estimate of riparian extent. Delineating riparian areas with buffers is appropriate only when compliance with a minimal level of regulation is required.
Click here for more information on buffers.