Riparian vegetation, topography, sediment quality, and river corridor geomorphology in the Lower Virgin River, Nevada and Arizona, before (2010) and after (2011-2012) a 40-year return period flood

The data release presents observations of riparian vegetation, topography, sediment quality, and river corridor geomorphology in four river reaches of the Lower Virgin River extending downstream 62 river kilometers (rkm) from near the town of Littlefield (AZ) and the Arizona-Nevada border at rkm 119. Methods included field observations and analysis of remotely-sensed data before (2010) and after (2011-2012) a 40-year return period flood (December 2010, at the gaging station “Virgin River near Littlefield” (USGS gage #09415000)).
The data release includes four .csv files related to field observations: UTM coordinates of field transect locations; vegetation and geomorphology; species codes; and sediment quality.  The data release also includes three shapefiles describing the river corridor geomorphology obtained through the interpretation of airborne color orthophotography: before the flood; after the flood; and the union of these two. Finally, there are four metadata (.xml) files – one associated with all four of the .csv files, and one for each of the shapefiles.
For the field surveys, in spring 2010, we established between 4 and 6 transects within each of the four study reaches. The total number of transects in the study area was 20. The transect locations were chosen to capture a representative range of geomorphological landforms (active channels, bars, floodplains, terraces) and existing riparian vegetation communities within each reach. Transects ran perpendicular to the active channel. Transect distances ranged between 131 to 585 m long, with each starting in the adjacent uplands and extending to at least the low flow channel. Along the field transects, vegetation was sampled in spring 2010 (pre-flood) and spring 2012 (post-flood) in 2 x 5 m (10 m2) plots located at 20-m intervals within each distinct geomorphological surface (distinguished by topographic position and dominant vegetation) along the length of each transect. The number of plots sampled was 385. Only 324 of these were re-sampled after the flood as they were submerged in the spring of 2012. Within each plot, a visual estimate of cover for each plant species present was recorded. To assess geomorphological change associated with the December 2010 flood, we used high-resolution topographic surveys (real-time kinematic (RTK) global positioning system (GPS)) along the transects before (spring 2010) and after (fall 2011, spring 2012) the flood. Sediment samples were collected once before the flood in spring of 2010 and once after the flood in spring of 2012, and a subset of 156 samples representative of all geomorphological surfaces was analyzed for texture (with only percentage in &gt; 63 µm-sand in fine fraction being reported) by hydrometer, and electrical conductivity (a measure of salinity) from a saturated paste extract (RC-16C Conductivity Bridge, Beckman Instruments, Brea, CA, USA).
The vegetation and geomorphology .csv file contains the cover of all species at the 709 observations, and the topographic information of each plot before and after the flood: elevation above water level, distance to the main channel, and a categorization of each plot based on change in elevation following the flood (erosion, deposition of no change, see definitions in the article associated to the dataset). The species codes .csv file contains the species abbreviations used in the vegetation .csv file. The sediment quality .csv file contains the analyzed sediment properties.
The river corridor geomorphology was described with a delimitation of the active channel before and after the flood using airborne color orthophotography collected in May–June 2010 (before flood) and in November 2011 (after flood). Working in a GIS environment, active channel areas were delineated based on the general absence of vegetation clearly observed in the pre- and post-flood imagery. Specifically, the active-channel areas are the distinct channel and floodplain areas obviously affected by flood flows (e.g., scoured to bare substrate), typically with less than 10% apparent remaining riparian vegetative cover (i.e., trees, shrubs, grasses). Barren areas obviously created by human land-uses were not included with the active-channel areas. The three shapefiles contain the delineation of the active channel before and after the flood, and their union.

Tags: arizona, biota, braided-channel, channel-widening, disturbance, large-flood, nevada, resilience, tamarix, usgs-5d76b653e4b0c4f70d01ff0d
Organization: Department of the Interior