"June 1979 4-R-37119." "Figure A.1." "Source: Lower Mississippi River Basin study, appendix LI." Available also through the Library of Congress Web site as a raster image.
United States. Soil Conservation Service
1979
<a rel="nofollow" class="external free" href="https://www.loc.gov/item/81692253/">https://www.loc.gov/item/81692253/</a>