Consistent Methods for Identifying Waters that may no longer be regulated under the Clean Water Act following the SWANCC decision.

Following the Supreme Court decision removing isolated waters from Clean Water Act jurisdiction, many states are being asked to identify the waters, particularly the wetlands, that are potentially no longer under Clean Water Act Jurisdiction.  “Isolated” is a not a scientific term and there is not a clear, simple breakdown that can be readily generated based on the Court’s decision.  The extent of the Court’s decision will remain unclear until the Federal agencies issue guidance defining how to apply a number of terms used by the court--such as adjacency, tributary, navigable, significant nexus and so on—to determine jurisdiction.  The way these terms are interpreted can add or subtract many millions of wetlands and other isolated waters from jurisdiction.  In the short term confusing and conflicting estimates of wetlands and other isolated waters affected can result.  These are not necessarily wrong, given the existing uncertainty, but the conflicting numbers will be based on different underlying assumptions and methodologies.  

The Association of State Wetland Managers is encouraging states to use consistent methods to produce these estimates and consistent explanations as well of the underlying assumptions used.  The instructions that follow lay out various likely scenarios for defining “isolated” waters.  If your organization has the capabilities to perform these analyses we would encourage you to follow these directions where possible, and to note deviation from these directions.  Using a standard such as this will result in comparable data generated across the country.  We encourage you to share comments and criticisms of these instructions as soon as possible to minimize the delay in producing useable data for future policy/legislative action.

Please perform as many of these scenarios as possible given your capabilities.  Carefully document your methods, and data used.  Contact James Robb at jrobb@dem.state.in.us 317-233-8802 with any questions or comments.

Scenario 1: Connection to stream (within positional accuracy of data)
Scenario 2: Proximity to connected waters
Scenario 3: Exclusion of intermittent/ephemeral streams as connections
Scenario 4: Inclusion of floodplain as a connection
Scenario 5: Inclusion of federal lands as a connection
Scenario 6: Presumption of groundwater connection unless impermeable soil present
Scenario 7:  Estimation of otherwise isolated waters that likely have groundwater connections

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Scenario 1: Connection to stream (within positional accuracy of data)

Step 1. Panel your NWI or State digital wetland vector data into a coverage.

Step 2. Dissolve all common wetland and deepwater boundaries. This will convert wetland and deepwater complexes into wetland/deepwater units.

Step 3. Intersect the wetland coverage with a hydrology coverage.  Note that the hydrology coverage may contain isolated water bodies such as lakes with no apparent outlet.  These need to be removed from the hydrology coverage before intersection.  One method of doing this is to remove all “L” designated reaches.  Those water bodies that are connected will be added back in through the buffered intersection.  Take into account the positional accuracy of the base data used when intersecting by assuming that polygons within the positional accuracy measure of the hydrology coverage are intersected by that coverage.

Step 4: Select those polygons that either intersect, or fall within the positional accuracy buffer of a state line, or US boundary.

Step 5: Select those polygons that fall within the positional accuracy buffer of the previously selected polygons.  Repeat until no new polygons are selected.  This should overcome the majority of any erroneously separated polygons.

Step 6: The remaining unselected units are potentially isolated wetlands/deepwater units.  The selected units are potentially non-isolated.

Step 7: Sum the wetland acreage for both isolated and non-isolated wetlands.  Sum the number of isolated and non-isolated wetland units.

Note: Identify your source of wetlands (i.e. National Wetlands Inventory or State Wetlands Inventory). Identify your hydrology layer (i.e. USGS 1:100,000 or 1:24,000 hydrology layer, National Hydrography Dataset (NHD) at 1:100,000 or 1:24,000, or State Hydrology layer). Buffer 1:100,000 scale hydrology data by 50 meters and buffer 1:24,000 data by 12 meters. Identify the width of the buffers used.  

Scenario 2: Proximity to connected waters

Following step 5 above, select all units that lie within 100-m of the previously selected units.  Repeat remaining steps.  Repeat with a 500-m buffer and 1,000- m buffer.

Scenario 3: Exclusion of intermittent/ephemeral streams as connections

Remove all intermittent and ephemeral streams from the hydrologic coverage before implementing step 3 above.  Repeat the remaining steps

Scenario 4: Inclusion of floodplain as a connection

Following step 4 above, select all units that are intersected by (within the positional accuracy of) the 100-year floodplain as described by FEMA.  Repeat remaining steps.  If data exists repeat for the 25-year or 50-year floodplains.

Scenario 5: Inclusion of federal lands as a connection

Following step 4 above, select all units that are intersected by (within the positional accuracy of) federal lands.  Repeat remaining steps.

Scenario 6: Presumption of groundwater connection unless impermeable soil present

Following step 4 above, select all units that are intersected by (within the positional accuracy of) NRCS soil polygons with high or moderate infiltration rates.  Repeat remaining steps.

Scenario 7:  Estimation of otherwise isolated waters that likely have groundwater connections

If the data exists, and it is appropriate to do so given limitations of the data, use digital elevation models, and well data to determine which of the otherwise isolated units likely come into contact with ground water.  Consider using the methods described by: Gerla, Philip J.  1999.  Estimating the Ground-Water contribution in Wetlands using Modeling and Digital Terrain Analysis. Wetlands. 19(2): 394-402.

Other possible scenarios:

There are a number of other possible links that may or may not recapture otherwise isolated wetlands.  Please feel free to come up with creative methods for estimating which waters may be recaptured by each of the following.  Please carefully document your methods so we may share them with colleagues across the country.

·        Connection through field tile, roadside ditches, or agricultural ditches

·        Recreational areas used by out of state travelers such as beaches, boat ramps, camping areas, parks, private hunting preserves, etc.

·        Areas used in interstate commerce such as mineral extraction, peat mining, muck mining, cranberries, wild rice, farmed wetlands, market fishing/clamming, drinking water, etc.

·        Land receiving federal subsidies or other federal financial assistance.

Data summary needs:

·        Area of wetlands and deepwater habitat isolated and total in each scenario.

·        Number of wetlands and deepwater habitat units isolated and total in each scenario.

·        Area of wetlands and deepwater habitat isolated in watersheds/subwatersheds with greater than 35% urban land use (or other appropriate indicator of imperviousness).

·        Acreage of wetlands in or near natural heritage sites, critical habitat, outstanding state or national resource waters, or other areas designated as ecologically sensitive or significant.