DOI: 10.5066/f77s7mzc
title: Orthophotography Classification and Ground Surveys to Understand Sandbar Formation, Geometry, and Persistence in the Lower Platte River in Nebraska, 2006-2014

description: Sandbars are ubiquitous in wide, braided, sand-bed rivers throughout the world. In the Great Plains of the United States, recovery and expansion of emergent sandbar habitat (ESH) has been a priority in lowland rivers where the natural extent of sandbars has been degraded.  Despite the importance of ESH to river ecology and management, quantitative observations of the deposition and erosion behaviors of populations of sandbars across long segments of braided rivers are rare.  This data release provides a series of those observations in support of a case-study of emergent sandbar deposition and erosion behaviors in the Platte River in eastern Nebraska, a wide, sandy, braided river at the eastern margin of the Great Plains, United States. The study area is the 163 kilometer segment of the Lower Platte River downstream from the Loup River power outfall to the mouth.  Two primary datasets were used.  The first dataset, PLATTEclass_ARCHIVE.gdb ESRI geodatabase, was derived from remotely-sensed data and was used in the classification of the active channel in 2006, 2010, and 2014.  The second dataset, SBerosion_ARCHIVE.gdb ESRI geodatabase, was derived from landbased assessments of sandbars and was used to characterize sandbar erosion.  Each geodatabase is packaged into a zip file that includes the geodatabase as well as corresponding metadata files that provide documentation for each dataset.  A synopsis of the contents within each geodatabase follows.

The PLATTEclass_ARCHIVE.gdb geodatabase contains shapefiles and raster files used for classification and analysis of the Platte River active channel from Columbus, Nebr. to the mouth, 2006-2014.  The geodatabase includes three raster files that describe the classification of active channel area in the Lower Platte River from Columbus, Nebr. to just upstream of the confluence with the Missouri River in 2006, 2010, and 2014. The active channel area is classified into four categories: 1- water, 2- emergent sand, 3- vegetation, and 4- unclassified. The classification approach is further documented in Alexander and others (2019) and the corresponding metadata files within this data release (CLASSplatte&lt;year&gt;_FGDC.xml, where &lt;year&gt; represents 2006, 2010, or 2014).  Extraction cells (not included in this data release) were developed to summarize the classification data into polygons for every 500-meter long subareas within the study area.  The development of those extraction cells as well as the compilation of classification polygons (given by CLASS&lt;year&gt;_poly polygon files) is described in further detail in the EXTcell_COM&lt;year&gt;_FGDC.xml files.    The geodatabase also contains a stream centerline file (described in USGS2009_CENTERLINEroute_FGDC.xml) that was used in developing the extraction cells and for representing stream distances relative to the confluence with the Missouri River. Center of mass points (centroid) were calculated for each extraction cell for use in characterizing sinuosity and are described in greater detail in the EXTcell_COM&lt;year&gt;_FGDC.xml files. High flow conditions in the lower portion of the reach during the 2010 image acquisition led to the inundation of sandbars that would otherwise be visible during normal flows.  As a result, the geodatabase also includes two  other shapefiles of on the ground surveyed sandbars from April 2011 that were used to supplement the orthophotography analysis that are described in greater detail within the SURV1_smalBARSid_FGDC.xml and UNION_SURV1_FGDC.xml files.

The SBerosion_ARCHIVE.gbd geodatabase contains sandbar datasets that were used to quantify the persistence, erosion, or deformation of emergent sandbars on a seasonal and within-season basis.  The geodatabase contains 15 shapefiles total, and a corresponding 15 metadata (.xml) files are included to better describe the contents of each shapefile.  Files include areal and geographical information for surveyed sandbars in the Lower Platte River in Nebraska.  Emergent sandbars greater than 2 acres were considered large bars and were surveyed by walking the perimeter above the daily high-water mark with a Magellan Mobile Mapper CX, which logged waypoints approximately every 3 meters (Alexander and others, 2013). The data and corresponding metadata associated with these large bars include the phrase &iuml;&iquest;&frac12;LARGEbars&iuml;&iquest;&frac12; in their filenames.  The most upstream and downstream ends of the large bars were extracted for the surveys of 2011 and 2012.  The data and corresponding metadata associated with these ends include the phrase &iuml;&iquest;&frac12;BARends&iuml;&iquest;&frac12; in their filenames. Emergent sandbars less than 2 acres were considered small bars and were visually assessed to estimate the center location and area of each using a laser rangefinder and a Magellan Mobile Mapper CX. The data and corresponding metadata associated with these small bars include the phrase &iuml;&iquest;&frac12;SMALLbars&iuml;&iquest;&frac12; in their filename.

There were four surveys associated with quantifying seasonal persistence, erosion, or deformation of emergent sandbars, these are grouped as feature dataset SEASONerosion_2011_2012. These shapefile names begin with 'SURV*' where '*' is a placeholder for the survey number ranging from 1 to 4. Survey 1 occurred from April 4 to 12, 2011; survey 2 occurred from July 26 to August 2, 2011; survey 3 occurred from October 31 to November 4, 2011; survey 4 occurred from April 24 to May 3, 2012.

There were two surveys associated with quantifying within-season persistence, erosion, or deformation of emergent sandbars, these are grouped as feature dataset SPATIALerosion_2014. These shapefile names begin with 'SURV*', where '*' is a placeholder for the survey number ranging from 7 to 8. Survey 7 occurred from July 10 to 25, 2014; survey 8 occurred from September 4 to September 7, 2014. The geodatabase also contains a stream centerline line file (described in USGS2009_CENTERLINEroute_FGDC.xml) for representing the Lower Platte River.