All drains have some affect on an adjacent wetland. The question, then, concerns what is an acceptable negligible hydrologic effect on the wetland. To answer this, various lateral effect or scope and effect equations have been developed to estimate the extent that a drain will lower the adjacent water table. The term lateral effect is variously defined by different wetland-related programs. The Natural Resource Conservation Service (NRCS) defines lateral effect as the distance on either side of a ditch or tile line within which wetland hydrology would be impacted by the installation of the ditch or tile line such that it may result in loss of eligibility for USDA program benefits. Some have defined it as the width of a strip of land drained such that it no longer meets the wetland hydrology criteria set forth in the 1987 Corps of Engineers Wetland Delineation Manual. For wetland regulatory purposes in Minnesota, lateral effect is defined more broadly as the effect of a drain on the adjacent water table. Estimates of these effects can sometimes be used to evaluate whether or not a drain has or will cause an unacceptable loss of wetland hydrology.
A series of setback tables have been developed and refined over time by NRCS to estimate the lateral effect of various drains in different soil types. These tables have become the standard for estimating lateral effect in Minnesota. BWSR in coordination with the St. Paul District Army Corps of Engineers (Corps) has developed specific guidance on the use of these tables in relation to wetland regulatory programs and wetland delineation manuals as follows. Note that the links within this 2013 document are no longer operable, but the guidance is still applicable aside from the clarification below.
BWSR Guidance Concerning NRCS Developed Drainage Setback Tables (pdf)
Subsequent revisions and refinements to setback tables has necessitated further clarification on assessing lateral effect for wetland regulatory purposes. Users should first go to the Minnesota NRCS Engineering website and select the Lateral Effect Distances link. You must download the Lateral Effect Database Tool and open it using Microsoft Excel software program. Follow the directions on the website and you will be able to enter County and mapped soil series which will generate the lateral effect distance. For some soil types (e.g. soils known to have high organic content) users will be prompted to seek further assistance from NRCS staff. In those instances, users should not seek NRCS staff assistance and should instead use the setback distance in the 2012 drainage setback tables that BWSR has adopted. NRCS can only provide technical support for USDA program eligibility, not the Wetland Conservation Act. The 2013 BWSR drainage setback guidance applies regardless of which tables are used.
2012 Drainage Setback Tables by County (use only if no value given in current NRCS tables per above)