Stream Buffer Rule = Cleaner Water, Little Cost
Iowa's Low Hanging Fruit: Appendix – Methodology
Delineating the Waterway
The USGS National Hydrography Dataset flow lines were used to identify surface water for the five Iowa counties. The data was filtered to show only features with a proper name (GNIS Name) in an effort to reduce the amount of ephemeral waterways assessed. The remaining flow lines were used as a guide for heads-up digitization of the shorelines based on recent (2007, 2009, 2010) high-resolution aerial photography. Computer-modeled identification of the surface water was also deployed using the same imagery. To be considered in the analysis the digitized surface water must have maintained an established bed and bank visible on the 2013 imagery, to filter out all ephemeral waterways.
Agricultural Streamside Buffers
After a representative footprint of all named waterways was defined, streamside buffers zones of 35, 50 and 75 feet were mapped. USDA Common Land Unit data were used to isolate only the buffers on land classified as agricultural. These areas were then assessed for the presence of vegetation using the Normalized Difference Vegetation Index, a product developed from the aerial photography. Every mile of assessed shoreline with a streamside buffer segment that didn’t meet the various standards was manually checked against 2013 imagery. Segments with streamside buffers narrower than the standards were also cross-tabulated with trout streams, highly erodible land tracts and impaired waterways to generate summary statistics.
The most recent land ownership GIS data (2014) were purchased from each the five counties. These data were all reclassified to identify parcels classified as “agricultural” and aggregated by unique landowner. Those parcels were then cross-tabulated with the EWG- assessed waterways and buffer areas to provide summary statistics by landowner and land classification.