Broken Stream Banks
Failure to maintain buffer zones worsens farm pollution
EWG Focused on 37 Southern Counties
Broken Stream Banks: EWG Focused on 37 Southern Counties
We focused our assessment on the 37 southern Minnesota counties for which recent high-resolution aerial photography is available.23 These counties account for roughly 57 percent of all acres of row crops in the state, according to the U.S. Department of Agriculture’s (USDA) 2012 cropland GIS data layer. In 13 counties, row crops occupy more than 75 percent of the land area. In the 30 counties, row crops take up more than 50 percent.
EWG analyzed high-resolution color infrared imagery taken in 2011 to determine what percentage of the perennial rivers and streams in the Minnesota Public Waters Inventory GIS layer were actually bordered by some or all of the required 50-foot buffers (See Box 1). We limited the analysis to rivers and streams that are longer than 1 mile, which account for 97 percent of the 8,649 acres of required buffers in the counties studied.
We manually checked every mile of perennial rivers and streams for errors in our analysis by comparing the 2011 remotely sensed GIS data outputs with 2012 or 2013 imagery. If we found that buffers that were listed as missing in 2011 were present in 2012 or 2013, we lowered the estimate of missing buffer acreage accordingly. If we found, however, that buffers present in 2011 were missing in 2012 or 2013, we did not increase our estimates of missing buffer acres. This means our results likely underestimate the extent to which cropland is currently encroaching on the required buffers.
EWG also compared the Center for Environmental Advocacy’s findings to our own results for a smaller set of perennial rivers and streams in the Middle Minnesota Watershed and found that in 82 percent of the cases, the 2008 and 2011 results were the same.
Click on the map to see which perennial rivers and streams in each watershed lacked the required buffers. (Figure 2)
Figure 2: Perennial rivers and streams are far better protected in some watersheds than in others.
Click on a colored watershed to see how well stream banks are protected.
Minnesota’s Public Waters Inventory
The Minnesota Department of Natural Resources (MDNR) defines “public watercourses” as natural or altered waterways that drain an area greater than two square miles and have a definable bed and banks, as specified in Minnesota Statute, section 103G.005. The department maintains a Public Water Inventory (PWI) of all public waters, watercourses and basins – including perennial and intermittent rivers, streams and drainage ditches – under its regulatory jurisdiction.
In 2008, watercourses identified on the inventory’s paper maps were digitized into a GIS layer. This GIS layer made EWG’s assessment possible.
Not all public drainage ditches are accurately mapped, because authorities around the state operate differently and have varying levels of record and map quality. As a result, the Inventory’s GIS layer does not include all public ditches. The Inventory also includes intermittent streams because they meet the criteria written in the statute and therefore fall under the same jurisdiction as perennial waterways.
Source: Minnesota Department of Natural Resources 2008. Minnesota Public Waters Inventory. http://deli.dnr.state.mn.us/metadata.html?id=L390006590202. Accessed March 1, 2013. St. Paul, MN
Figure 3: Three Types of Public Waterways in Southern Minnesota
EWG assessed the presence or absence of buffers along all the perennial rivers and streams in the Inventory’s GIS layer that were adjacent to cropland. We only assessed a small subsample of the ditches and intermittent streams in the Inventory that were associated with cropland. The perennial rivers and streams we assessed make up 56 percent – about 8,000 miles – of all public waterways in agricultural areas of the 37 counties studied. The intermittent streams and ditches we did not assess comprehensively account for 27 and 17 percent respectively of all waterways in agricultural areas on the PWI GIS layer. (Figure 3)
The distribution of different categories of waterways on the Inventory’s GIS layer varies from county to county. Some have very few perennial rivers and streams in agricultural areas that are longer than one mile. The grey areas in the map in Figure 2 are watersheds with no such perennial rivers and streams longer than one mile. Similarly, some counties have many miles of intermittent streams and ditches, while others have very few.
Table 1: Only 18 percent of perennial river and stream banks in agricultural areas are fully protected.
|Grade||Percent of Required Buffer Present||Missing Buffer Acres||Number of Waterways||Percent of Waterways|
Figure 4: Worst protected perennial waterways are concentrated in four areas.
Figure 5: Four examples of what we found along perennial waterways.
Overall, EWG found that far too many southern Minnesota waterways lack all or part of the required 50-foot wide buffer zones between cropland and the stream bank. Buffers were most likely to be missing along the smaller perennial streams that are closely integrated with agricultural land. Drainage ditches and intermittent streams in our subsample had even fewer buffers.
EWG graded waterways using the traditional A-B- C-D-F system based on how much of the required 50-foot buffer acres were present, according to this system:
A – 100 percent of the required acres
A minus – 90-to-99 percent
B – 80-to-89 percent
C – 70-to-79 percent
D – 60-to-69 percent
F – less than 59 percent