Stressor Indicators

watershed % steep slope agriculture

Why relevant to recovery: See watershed % agriculture for rationale.  Specifically on steep slopes, cropland is associated with higher erosion and overland transport of nutrients and other pollutants.  Abundant steep-slope agriculture in a watershed may represent a significant difficulty to overcome in restoration efforts.

Data sources and measurement: Calculated as % by area within watershed; land cover data and elevation data (preferably converted to slope map categories) are necessary to target the steep slope agriculture. Land cover sources include the National Land Cover Data from 1992 (See: http://www.epa.gov/mrlc/nlcd.html), 2001 (See: http://www.epa.gov/mrlc/nlcd-2001.html), and 2006 (http://www.mrlc.gov/nlcd06_data.php ) as well as various state sources. If the user chooses to use this indicator for a specific crop relevant to the study area, USDA has developed a national GIS crop dataset that can be downloaded from Geospatial Data Gateway (See: http://datagateway.nrcs.usda.gov/GDGHome.aspx ). The National Elevation Dataset (NED) (See: http://nhd.usgs.gov/index.html ) is adequate for generalized differences in elevation. High resolution elevation data should be used for any assessment units at HUC12 level of smaller.  The Elevation Derivatives for National Applications (EDNA) has been derived from the NED and is hydrologically conditioned to improve hydrologic flow representation (see: http://edna.usgs.gov/ ).  NHD plus contains information on maximum and minimum elevation for each flowline (http://www.horizon-systems.com/nhdplus/ ).

watershed number of septic systems

Why relevant to recovery: Although subject to regulations, private septic systems are in failure frequently enough that many watershed models routinely assume a "% septic failure" factor of 30%.  or more. Failed septics that reach waterways can increase nutrient and pathogen loadings, as well as deliver household toxins such as chlorine. Because of difficulty of detection, failed septic systems are also an obstacle to effective recovery and restoration targeting.

Data sources and measurement: Land cover maps overlaid with non-sewered area maps can help identify zones of potential septic usage and assumed partial failure rates. Some municipalities have individual septic records that can be summed by township or watershed. Local watershed studies and TMDLs may provide a source for septic failure coefficient assumed in the area.

watershed % tile-drained cropland (PDF) (3 pp, 51.9K, About PDF)

Why relevant to recovery: Tiles efficiently drain water from the soil saturated zones to streams, thereby reducing residence time in areas conducive to denitrification and increasing nitrogen export. Tile draining also has created concern for the delivery of sediment, sources of bacteria, contaminants, and suspended solids. Tile drains can selectively transport fine-grained sediment from soils to receiving freshwater, increase the size of the contributing area by hydraulically connecting remote areas of the catchment to the stream system, and circumvent management strategies such as buffer strips. Subsurface drain tiling that accompanies wetland drainage can lead to flashy hydrology that can decimate the stream biota. Most of the above effects are exacerbated by the way tiles extend the total area producing these negative impacts farther out into the watershed.

Data sources and measurement: Based on verifying the association of tile drain usage with specific hydric soil types that are being cropped. Basic information needs include three elements: locally gathered monitoring data on agricultural practices associated with specific hydric soil types and agricultural uses (e.g., NASS, NRI surveys by USDA), soil type mapping, and agricultural land cover mapping). Digital soil survey data varies from State to State in availability. States with fully digitized county soil survey-level information can use this metric most effectively. Physical and chemical properties of soils are available for most areas as part of the US General Soils Map through the NRCS Soil Data Mart (See: http://soildatamart.nrcs.usda.gov/ ). Land cover sources include the National Land Cover Data from 1992 (See: http://www.epa.gov/mrlc/nlcd.html), 2001 (See: http://www.epa.gov/mrlc/nlcd-2001.html), and 2006 (http://www.mrlc.gov/nlcd06_data.php ), as well as various state sources. The USGS lists cropland by county since 1850 (See: http://landcover.usgs.gov/cropland/index.php ). If the user chooses to use this indicator for a specific crop relevant to the study area, USDA has developed a national GIS crop dataset that can be downloaded from Geospatial Data Gateway (See: http://datagateway.nrcs.usda.gov/GDGHome.aspx ).

watershed % urban (PDF) (30 pp, 311K, About PDF)

Why relevant to recovery: Urbanization of a watershed results in multiple stressors to a watershed, such as increased pollutant loads from stormwater runoff, altered stream flow, decreased bank stability, and increased water temperatures. The significance of this metric in reducing recovery potential is based on the multiple impacts to the watershed as well as the nearly irreversible nature of imperviousness. (See also Watershed Impervious Cover under Stressor Exposure indicators.)

Data sources and measurement: Measured as a percent of the area of a watershed with a land use classification of "urban" (i.e. low, medium, and high density residential; commercial; industrial; etc). For land cover data, the National Land Cover Database (NLCD) for 2006, 2001 and 1992 is accessible at http://www.mrlc.gov/finddata.php ; numerous statewide land cover mapping datasets are also available from state-specific sources.

other % watershed stressor

Why relevant to recovery: The indicators described in this table are not an exhaustive list of all the stressors that might play a highly significant role in the restorability of some watersheds. Surface mining, for example, is not among the indicators summarized but can play a major role in comparison of recovery potential of some watersheds. This metric is a simple placeholder for regionally-significant stressors in watersheds that are not captured in other indicators.

Data sources and measurement: As appropriate, depending upon data source.

corridor % impervious cover (PDF) (3 pp, 50.9K, About PDF)

Why relevant to recovery: Impervious cover is an indicator of the impacts of urbanization and development on water resources. Some literature reveals greater impacts of corridor urbanization and imperviousness than the same activities across the watershed. Impervious cover results in multiple stressors to a watershed, such as increased pollutant loads from stormwater runoff, altered stream flow, decreased bank stability, and increased water temperatures. The significance of this metric in reducing recovery potential is based on the multiple secondary impacts to the corridor and water body as well as the nearly irreversible nature of imperviousness. (See also Watershed Percent Impervious.)

Data sources and measurement: Multiply the area classified as "urban" (i.e. low, medium, and high density residential; commercial; industrial; etc) within a defined corridor width (e.g. 90 meters per side) by the appropriate impervious cover coefficient for each land use type. The percent impervious cover indicator is calculated by averaging the impervious cover areas across the total land area of the corridor. If possible, differentiating between impervious cover contiguous with or isolated from drainage should be done to estimate 'effective' impervious cover. The 2001 and 2006 National Land Cover Data contains information on impervious covers as well as urban land cover (See: http://www.epa.gov/mrlc/nlcd-2001.html  and http://www.mrlc.gov/nlcd06_data.php ). Approximate watershed boundaries can be constructed by aggregating small-scale catchments from the NHDplus datasets (See: http://www.horizon-systems.com/nhdplus/ ).

corridor % U-index  (PDF) (2 pp, 42.2K, About PDF)

Why relevant to recovery: Both riparian and watershed-wide U-index (anthropogenic) land cover patterns are associated with benthic macroinvertebrate communities that are tolerant of stream degradation, indicating a lower level of aquatic ecological integrity and water quality. As the intensity of human activities increase there is a tendency that the biological integrity of the rivers decreases. Increasing substrate embeddedness and bank erosion have also been observed to increase in streams in developing areas. High riparian U-index may indicate that, as widespread anthropogenic cover is unlikely to be reduced and is complex to remediate, U-index may be a strong determinant of poor recovery prospects.

Data sources and measurement: Extracted from land cover mapping within a set corridor width, and summarized as % anthropogenic cover types (e.g. developed, agricultural) by area within the corridor. For land cover data, the National Land Cover Database (NLCD) for 2006, 2001 and 1992 is accessible at http://www.mrlc.gov/finddata.php ; numerous statewide land cover mapping datasets are also available from state-specific sources. This metric can also be measured with a narrow corridor width or as a linear feature to identify specifically the land-water interface proportions that are in human-altered cover types.

corridor % agriculture (PDF) (5 pp, 51.9K, About PDF)

Why relevant to recovery: Croplands and pastures have been linked to a wide variety of water quality and biotic impacts on waters. Agriculture within stream corridors is sometimes more highly linked with several impairment types than agriculture generally distributed in the watershed, but often watershed percentage also appears to be a strong influence. Common effects seen at moderate to high agricultural proportions of land cover include less diverse and more intolerant macrobenthic communities, increased nutrient loading resulting in turbid water, overall homogenization of the fish fauna, accelerated erosion and bank destabilization, suspended sediment particles carrying pesticides, pathogens, and heavy metals, habitat degradation and reduced biodiversity, and increases in specific conductivity, DIN, DRP, and TP concentrations. Although agriculture is commonly linked to degraded aquatic conditions that may be difficult to reverse and quite persistent over time, it is important to note that some degree of recovery is rarely considered impossible; for example, livestock access to channels may be more influential than corridor agricultural use and is commonly feasible to reduce. However, some studies claim that agriculture on floodplains can constrain the benefits of restoring natural hydrologic processes.

Data sources and measurement: Calculated as % by area within a set corridor width (e.g., 30 M or 90 M on each side); often cropland and pasture are compiled as separate metrics. Land cover sources include the National Land Cover Data from 1992 (See: http://www.epa.gov/mrlc/nlcd.html), 2001 (See: http://www.epa.gov/mrlc/nlcd-2001.html), and 2006 (http://www.mrlc.gov/nlcd06_data.php ) as well as various state sources. The USGS lists cropland by county since 1850 (See: http://landcover.usgs.gov/cropland/index.php ). If the user chooses to use this indicator for a specific crop relevant to the study area, USDA has developed a national GIS crop dataset that can be downloaded from Geospatial Data Gateway (See: http://datagateway.nrcs.usda.gov/GDGHome.aspx ). In addition, where applicable, BLM data set on range allotments and pastures can be used (See: http://www.geocommunicator.gov/GeoComm/ ).

corridor road crossings (PDF) (2 pp, 43.3K, About PDF)

Why relevant to recovery: Road crossings are linked with degraded condition for several reasons, but because most crossings are likely permanent, their relevance to recovery potential is also linked to whether the degraded conditions can be managed or mitigated for. Road crossings are linked with channel destabilization, tree collapse, hanging tributary junctions as a result of variable incision rates, and erosion around artificial structures including bridges. Local scouring alters sedimentation and deposition processes, and more sediment and chemicals enter streams where a road crosses. Wetland road crossings often block drainage passages and groundwater flows, effectively raising the upslope water table and killing vegetation by root inundation, while lowering the downslope water table. Small road crossings for which the culverts do not allow upstream fish passage and constrict the available useful habitat for salmonids already vulnerable to other impacts.

Data sources and measurement: Measured as number of crossings per stream mile. Land cover or transportation GIS data are widely available. National road and stream data is obtainable through the National Atlas (See: http://nationalatlas.gov/ ). Landsat data is also often used for road and stream data and can be accessed through the USGS Earth Explorer (See: http://edcsns17.cr.usgs.gov/EarthExplorer/ ). ESRI offers a free roads dataset (http://www.arcgis.com/home/item.html?id=3b93337983e9436f8db950e38a8629af ). Data on unimproved road crossings in remote parts of federal lands may need to be obtained through the land management agency.

channelization (PDF) (4 pp, 126K, About PDF)

Why relevant to recovery: Channelization is a major modification of natural form that results in habitat simplification and reduction in frequency of specific, life-supporting habitat types (e.g. pools, spawning gravels). The process also destabilizes erosion/deposition dynamics, shortens residence time during which excess nutrients may be processed, and increases risks of downstream erosion and channel destabilization with accompanying loss of use or property. Negative impacts on biological communities are well documented not only within channelized reaches but at substantial distances downstream. The significance of this metric in reducing recovery potential is based on multiple effects: degraded habitat, altered primary physical processes, destabilized instream conditions, persistence of negative effects for decades, and high expense of reengineering channel sinuosity.

Data sources and measurement: The simplest manner of measuring channelization is as the percent of total impaired segment length that is artificially straightened, as observable on imagery or mapped media. Straight channels are easily detected, but highly reduced sinuosity (which often has similar effects) is less easily identified unless factors such as valley slope and expected vs observed sinuosity are measured. Additional measurements may choose to consider the channelized length per watershed area, whether lined or armored banks or bed are present, and percent of channelized length weighted by Strahler stream order. A visual inspection of these data is the best way to identify presence/absence of channelization, after which measurement of length channelized and impaired 303(d) reach length can be carried out via GIS to obtain the % channelized and total channelized length. Except on very short reaches or the smallest channel orders, much channelization is visible on high resolution surface hydrography data such as data available through the National Hydrography Dataset (See: http://nhd.usgs.gov/ data.html) or local resources. Manual visual interpretation from GIS hydrographic data, although somewhat laborious, is effective for identifying and measuring the length/percent of channelized reaches in each impaired waterbody segment. Channelization presence/absence is sometimes reported as a cause for 303(d) listing and available as attribute data from EPA's ATTAINS data system (See: http://www.epa.gov/waters/ir/).

relative net water demand

Why relevant to recovery: Stressors affecting the natural flow regime can have numerous secondary impacts. Ecological responses to flow alterations include loss of sensitive species, reduced diversity, altered assemblages and dominant taxa, reduced abundance, and increases in non-native species. Human water use can affect the magnitude, frequency, and duration of streamflows. For this metric, ratios of impacted streamflow to natural streamflow are used as indices to describe the magnitude of hydrologic alteration. Specifically, in this USGS-developed metric annual withdrawal and discharge are averaged and disaggregated to a one-year time series of daily flow. Net daily water use is subtracted from natural streamflow to get impacted streamflow for the period of record.

Data sources and measurement: Flow information is usually limited, but gaging station records can sometimes be used to develop natural flow estimators and calculate this metric relative to natural flow.

invasive species risk (PDF) (5 pp, 77K, About PDF)

Why relevant to recovery: Non-indigenous species (NIS) invasions are widely known to disrupt aquatic system function and inhibit recovery of altered systems. The rapid colonization typical of NIS may act to subvert expected succession pathways and thereby disrupt restoration planning. Altered structure due to aquatic or riparian NIS can reduce shade, inhibit native riparian vegetation cover, and increase sedimentation. Aquatic invaders may compete directly or prey upon key native species, reduce numbers or species diversity, and markedly alter food webs and ecological structure. Presence of NIS may actually be the impairment cause for listing, and recovery in such cases depends on eradication or control. Particularly relevant to recovery potential screening is the fact that some NIS, once established, cannot currently be controlled or eradicated by any known methods.

Data sources and measurement: In recovery potential screening, this metric may consider existing invasions or the risk of future invasions, or both. Many options for scoring can be developed, based on the state. An example quantile scoring process is:

• 0 - no established NIS of concern, no immediate risk;
• 1 - no established NIS of concern, risk due to proximity or other vulnerability;
• 2 - established NIS of concern exists, control or eradication feasible;
• 3 - established NIS of concern exists, control or eradication infeasible.

Data availability may be through waterbody-specific monitoring information on occurrence, such as the USGS Non-Indigenous Aquatic Species Information Resource (See: http://nas.er.usgs.gov/default.aspx) or through Non-Indigenous Species Database Network range maps by species (http://www.nisbase.org/nisbase/index.jsp ) which include a variety of participating databases. The USDA National Invasive Species Information Center also contains links to several databases (See: http://www.invasivespeciesinfo.gov/resources/databases.shtml ). Availability of either is highly variable and difficult to update under rapid changes.

number of 303d listed causes (PDF) (1 pp, 11.9K, About PDF)

Why relevant to recovery: The number of stressors affecting and impaired water body is generally a direct indication of the relative complexity, expense and difficulty of its restoration, according to many practitioners. More pollutants causing impairments frequently implies more numbers and diverse types of responsible sources. The number of listing causes also may be associated with greater magnitude of impairment due to cumulative effects.

Data sources and measurement: Number of pollutant causes per listed water body segment, or number of listed cause/waterbody segment combinations per watershed, which are both identifiable from EPA data systems available online. If the reporting unit contains more than one listed waterbody segment, the total number of pollutant causes per length of listed waterbodies can be measured. The Assessment TMDL Tracking and Implementation System (ATTAINS) (See: http://www.epa.gov/waters/ir/) contains information on 303d-listed waters by state and by semi-annual reporting cycle. States may also have more detailed information.

CSO or MS4 areas

Why relevant to recovery: Public sewer systems are designed to handle stormwater runoff and its pollutants, up to a point. Exceeding the capacity of a system can result in an episodic increase in pollutant loadings and complicate efforts at restoration. Existence of CSO or MS4 areas in a watershed as well as the capacity of the systems can be a consideration when evaluating stressors that affect recovery.

Data sources and measurement: Generally these areas are available in mapped form at state or municipal level.

severity of loading (PDF) (2 pp, 15K, About PDF)

Why relevant to recovery: For impaired waters where needed load reductions have been calculated, the magnitude of necessary reductions compared with current loadings has been shown to relate to likelihood of successful restoration, although this metric is not necessarily a determinant of irreversible degradation. Case studies of restoration successes showed multiple cases where if needed load reductions were less than 50% of current levels, more restoration successes were achieved. The 50% figure is likely not a consistent threshold value and data of this sort are limited, thus the metric is best used to array a set of waters into quantiles based on expert judgment about % loading reduction.

Data sources and measurement: Data sources and measurement: The measure compares the current loading estimates with the TMDL target loading calculation, in terms of the percent reduction needed. The Assessment TMDL Tracking and Implementation System (ATTAINS) (See: http://www.epa.gov/waters/ir/) contains information on 303(d)-listed waters by state and by semi-annual reporting cycle. Loading estimates generally need to come from completed TMDLs or watershed models. Most completed TMDLs are available online via state or EPA websites (see also http://www.epa.gov/waters/tmdl/expert_query.html).

SPARROW nitrogen loading estimate

Why relevant to recovery: SPARROW is one of the most widely used geospatial models for estimating nutrient pollution on a watershed basis, and has been used to estimate nitrogen and phosphorus loads and yields over large areas at the HUC8 and HUC12 scales. These models identify urban and agricultural sources as major contributors of nutrients to streams and reveal local and regional differences in nutrient contributions from contrasting types of agricultural (farm fertilizers vs. animal manure) and urban (wastewater vs diffuse runoff from developed land) sources.

Data sources and measurement: The EPA's NPDAT website (see http://www.epa.gov/nutrientpollution/npdat/) provides an introductory website, geospatial viewer, and data downloads as well as associated water quality information.  An online, interactive USGS decision support system  also provides easy access to regional models describing how rivers receive and transport nutrients from natural and human sources to sensitive waters, such as the Gulf of Mexico.

watershed stream miles impaired

Why relevant to recovery: Although state monitoring programs are generally unable to assess all of their waters in each integrated reporting cycle, the relative quantity of reported impairments provides an important insight into what is currently known. Larger quantities or proportions of impaired stream miles imply a likely more complex and extensive watershed restoration task, and also may be associated with additional unmonitored impairments within adjacent or connected streams and other water bodies.

Data sources and measurement: This metric can be used as a quantity (total impaired miles per watershed) or as a proportion (impaired miles per total stream miles in the watershed, impaired miles per total watershed miles assessed). A national geospatial impaired waters dataset is available through EPA's Assessment TMDL Tracking and Implementation System (ATTAINS) (See: http://www.epa.gov/waters/ir/). This source contains information on 303(d)-listed waters by state and by semi-annual reporting cycle.

modeled watershed aerial N deposition

Why relevant to recovery: Excessive N loadings to waters cause a number of adverse effects, whether from land-based sources or aerial deposition. Aerial deposition is worth separate consideration as a factor inhibiting recovery potential as the likelihood of successful control of the aerial sources is independent of the likelihood of controlling the within-watershed point or nonpoint sources of N. Where aerial sources continue to be the primary sources of N, watershed-based restoration efforts alone might be expected to have low recovery potential.

Data sources and measurement: Data sources and measurement: Modeled N deposition across large regions may be available for selected areas at a resolution that enables aggregation into N deposition values at a given watershed scale, e.g., HUC12s. Likely that original model outputs are not on a watershed basis, but areal weighting methods allow for translating values to a watershed basis.

other stressor-specific severity factors

Why relevant to recovery: The broad variety of impairment causes that can affect degraded waters may vary substantially from one another in relative difficulty of restoration. A given area may, for example, find their urban runoff impairments far more difficult to remediate than their rural pathogen impairments. This metric relates to differentiating between specific stressors and their generalized differences in restorability as a recovery potential indicator concept. This concept can also be applied to develop single-stressor-based indicators that recognize recovery potential differences related to loading magnitude, frequency, duration, or association with other factors that influence restorability.

Data sources and measurement: Project-specific and stressor-specific measurement methods need to be developed to use this indicator.

land use change trajectory (PDF) (4 pp, 72.6K, About PDF)

Why relevant to recovery: Human use effects in watersheds that influence recovery can be observed not just by current conditions, but also by recent changes in those conditions. Recent land cover trajectory studies may suggest the likely direction of continued pressures, such as continuing urbanization, deforestation, or agricultural expansion. Also, recent changes may not have fully produced impacts that can occur over several years. Some studies have suggested that decades-old land use history is sometimes more correlated with aquatic impairment than recent land use pattern.

Data sources and measurement: Land cover totals (%) for given watersheds can be contrasted for different past time periods. Buildout scenario projects may be sources of estimating future change trajectory, as are data sources that identify high-growth urban areas. Specific changes are more easily tracked than attempting to summarize all watershed change types in one metric - for example, one can separately estimate recent loss in % forest, gain in % urban, and gain in % agriculture in each watershed of interest. National Land Cover Data from 1992 (See: http://landcover.usgs.gov/natllandcover.php ), 2001 (See: http://www.mrlc.gov/index.php ), and 2006 (http://www.mrlc.gov/nlcd06_data.php ) can be contrasted at the watershed level. MRLC provides calculated data on developed imperviousness change, as well as the change for all of the NLCD land use classes, between 2001 and 2006 (http://www.mrlc.gov/nlcd06_data.php ). The USGS is a source for several land use change datasets, including the Land Cover Trends Project (See: http://landcovertrends.usgs.gov/download/overview.html ) and the Temporal Urban Mapping project (See: http://landcover.usgs.gov/urban/umap/ ). State or local data on land cover change are not common and may present technical challenges to data comparability over time. The Historical Topographic Map Collection includes published U.S. maps of all scales and editions, and are offered as a georeferenced digital download or as a scanned print from the USGS Store (see http://nationalmap.gov/historical/ ).

watershed % legacy agriculture (PDF) (5 pp, 69.5K, About PDF)

Why relevant to recovery: A variety of impacts on waters are associated with agricultural uses in general, including increased loadings of nutrients, fine sediment, pesticides, herbicides, flow alteration, elevated water temperature, and others. A past history of agriculture in the watershed and/or riparian corridor can continue to account for adverse effects even after land use change (vegetational succession, transition to residential or other uses) has replaced the agriculture. Built-up nutrients and pesticides/herbicides in groundwater can continue to be discharged through influent groundwater connections for decades. Excess fine sediments and channel widening (which reduces channel habitat quality) caused during active agriculture can also persist for years to decades, as can channel destabilization and related erosion. Some studies suggest that a past history of agricultural use is more strongly correlated with impairment than current, other land use patterns.

Data sources and measurement: Although geospatial data on past agriculture is date-limited, one national digital source (LUDA/GIRAS) characterized agricultural usage in the 1970s at coarse resolution. Locally older data may be available in isolated areas, or where historical aerial analyses back to the 1930s may have been conducted using airphotos. Measurable with suitable data as percent by area within watershed or corridor. Historical land cover data is available through the USGS Land Cover Institute (See: http://landcover.usgs.gov/cropland/index.php ). NLCD land cover data is available as far back as 1992 (http://landcover.usgs.gov/natllandcover.php ). MRLC provides calculated data on the change for all of the NLCD land use classes between 2001 and 2006 (http://www.mrlc.gov/nlcd06_data.php ).

corridor % legacy agriculture (PDF) (5 pp, 71.9K, About PDF)

Why relevant to recovery: A variety of impacts on waters are associated with agricultural uses in general, including increased loadings of nutrients, fine sediment, pesticides, herbicides, flow alteration, elevated water temperature, and others. A past history of agriculture in the watershed and/or riparian corridor can continue to account for adverse effects even after land use change (vegetational succession, transition to residential or other uses) has replaced the agriculture. Built-up nutrients and pesticides/herbicides in groundwater can continue to be discharged through influent groundwater connections for decades. Excess fine sediments and channel widening (which reduces channel habitat quality) caused during active agriculture can also persist for years to decades, as can channel destabilization and related erosion. Some studies suggest that a past history of agricultural use is more strongly correlated with impairment than current, other land use patterns.

Data sources and measurement: Although geospatial data on past agriculture is date-limited, one national digital source (LUDA/GIRAS) characterized agricultural usage in the 1970s at coarse resolution. Locally older data may be available in isolated areas, or where historical aerial analyses back to the 1930s may have been conducted using airphotos. Measurable with suitable data as percent by area within watershed or corridor. Historical land cover data is available through the USGS Land Cover Institute (See: http://landcover.usgs.gov/cropland/index.php ). NLCD land cover data is available as far back as 1992 (http://landcover.usgs.gov/natllandcover.php ). MRLC provides calculated data on the change for all of the NLCD land use classes between 2001 and 2006 (http://www.mrlc.gov/nlcd06_data.php ).