Troubleshoot sedimentation risk in NHDPlusV2 catchments

Multiple current issues, including:

1. Existing map as of current date is pretty dissimilar from the 2013 assessment.
- I have tried:
* Using watershed cumulative values instead of catchment-specific values. The resulting map is more dissimilar (highlights larger reaches as higher sedimentation risk).
* Binning the resulting sedimentation risk values in the visualization by each 10th percentile, matching the visualization of the 2013 assessment. This did not improve comparability.
- Still need to examine:
* Calculation of sedimentation risk in R: is there something funky going on with NAs or other values?
* I'm open to more suggestions!
3. Discussion of usefulness of sedimentation risk as binary variable. Sediment can be needed in a given reach, or be a pollutant/water impairment. So, this indicator may not be useful generally.

Start by locating the R calculation of sedimentation risk and the visualization that produces the current map. Check NA handling and the use of catchment-specific versus watershed cumulative values, then compare results with the 2013 assessment; done means explaining the discrepancy and documenting whether a binary sedimentation-risk indicator is useful.

Written by the indexing model from the issue text.