Valley Bottoms of all Perennial Streams: Colorado Plateau Ecoregion

The perennial 1:24,000 National Hydrography Dataset (NHD) stream network and 10m Digital Elevation Model (DEM) were used within the Fluvial Corridor Valley Bottom Tool, an ArcGIS 10.1 Tool Box, to delineate the valley bottom of all perennial streams within the entire Colorado Plateau Ecoregion (CPE) (Roux et al, 2014). Valley bottoms, as we define them, consist of the active channel and the modern alluvial floodplains (deposition zones of alluvium since Holocene). The Valley Bottom Tool generates a detrended elevation model used to compare ground and stream elevations which allows the user to “flood” the valleys to specified heights. Several runs of the tool were performed creating valley bottoms ranging from wide to very narrow. These various valley bottoms were used in the manual editing process. To enable effective editing, the ecoregion and associated valley bottoms was divided into Hydrologic Unit Code (HUC) 8 subsets. Valley bottom editing was conducted in ArcMap (10.2) at a scale of 1:10,000, however in confined valley settings and on small streams 1:8,000 or 1:6,000 scale was required to capture the required detail. The widest valley bottom portions of each HUC were analyzed to determine which valley bottom, from the tool described above, most closely resembled the actual valley bottom of these unconfined valley settings. This Valley bottom was used until there was a significant change in valley setting within the watershed. At the watershed scale, valley bottoms generally narrow as one moves upstream and approaches the headwaters. Consequently, it was necessary to change the valley bottom being edited from the wider version to a narrower, more suitable version. This was done by using the “cut polygon” tool to make a cut across the entire valley bottom being edited and then deleting the portion upstream from the cut. The narrower valley bottom was then edited and cut just downstream from where the wider valley bottom was cut. After starting an editing session on the original valley being edited again, it was possible to copy and paste the narrow valley bottom cut off for the upstream portion into the valley bottom being edited. In many cases a watersheds final valley bottom was a combination of three different sizes of valley bottoms, generated from the Valley Bottom Tool, beginning with a wider valley bottom in unconfined settings, a mid-sized valley in partly confined setting and a narrow valley in confined and headwater settings. In certain cases, the most suitable valley bottom shapefile for editing was generally slightly larger than the actual valley, and in other cases it was generally slightly smaller. Where it was larger, the shapefile was clipped down using the “cut polygon” editing tool, and where it was smaller it was added onto using the “auto complete polygon” editing tool. The most effective lines of evidence for determining the edges of the valley bottoms were satellite imagery, hillshades, and topographic map data. In most cases the physical features that confine valleys were fairly obvious using these layers. In cases where the physical features were smaller or vaguer, additional lines of evidence were used to help determine where to draw the lines. These included Federal Emergency Management Agency (FEMA) floodplain data, US Fish and Wildlife Service National Wetlands Inventory (NWI) wetland data, National Cooperative Soil Survey SSURGO full and partial hydric soil data, and various land cover data depicting wetland and riparian vegetation. In general, it could be concluded that the valley bottom would include the polygons in these layers, which made it easier to determine where to draw the lines in the absence of obvious features. In difficult areas Google Earth was also a very helpful resource and allowed zooming into an area of interest using the 3D view with vertical exaggeration added to visualize breaks in slope and other features that were not obvious in other data sets. Additionally, valley bottom shapefiles were converted to KML files which could be opened and observed in Google Earth, where any obvious flaws could be spotted and corrected. In confined valley settings the original valley delineations from the Fluvial Corridor tool were fairly accurate, and required only minor editing. In partly confined and unconfined valley settings, the raw data was less accurate, and required more editing. In addition, the valley bottoms of small headwater streams tended to be slightly over exaggerated because at the scale that we were performing the edits it was virtually impossible to clip these areas down anymore.