Freshwater Resilience, Highest and High, Watersheds for Complex and Non-complex Stream Networks, Northeast U.S

Freshwater Resilience, Highest and High, Watersheds for Complex and Non-complex Stream Networks, Northeast U.S. is one of a suite of products from the Nature’s Network project (naturesnetwork.org).

As growing human populations increase the pace of climate and land use changes, estimating the resilience of freshwater systems will be increasingly important for delivering effective long-term conservation.  A region-wide analysis of freshwater stream networks was developed by Mark Anderson and associates at The Nature Conservancy (Anderson et al., 2013) to estimate the capacity of each network to cope with climatic and environmental change. The analysis centered on the evaluation resiliency: characteristics that may allow stream ecosystems to maintain diversity and function within a dynamic climate, and that could be modeled in GIS with confidence at the regional scale and were not highly correlated with each other.  

Please find the full report at http://www.conservationgateway.org/ConservationByGeography/NorthAmerica/UnitedStates/edc/Documents/FW%20resilience_report_11_2013_distribute.docx

This dataset was derived from the analysis of freshwater resilience of stream networks in the Northeast and Mid-Atlantic region. A stream network was defined as a continuous system of functionally connected streams, rivers, and lakes bounded by dams or upper headwaters. To facilitate combination with other Nature’s Network products, stream networks were delineated into watersheds (roughly HUC 10/12), rather than stream networks, in this version.  

The high and highest relative resiliency watersheds identified in this product are those that, compared to other watersheds with similar native fish composition, rate higher than average on seven characteristics correlated with resilience. These included four physical properties (stream network length, number of size classes, number of gradients classes and number of temperature classes), and three condition characteristics (risk of hydrologic alterations, natural cover in the floodplain, and amount of impervious surface in the watershed). Because biota and physical processes are linked to the size of bodies of water, watersheds of stream networks which contain a variety of stream and lake sizes are expected to be more resilient by providing varied habitats and refugia, allowing native species to persist.

Watersheds with at least five size classes are highlighted as “complex” in this product. There was high correspondence between complex and high/highest relative resiliency-rated stream networks, and The Nature Conservancy’s portfolio of freshwater priority rivers (available as a separate dataset). A separate dataset which includes all stream networks across the full range of resiliency scores is also available, but note that stream networks do not correspond to HUC 12 watersheds used in other Nature’s Network aquatic products: https://nalcc.databasin.org/datasets/6fe82a3f1caa45aaafd65907abc4c38d

Intended Uses

This dataset is included in the Nature’s Network package as an overlay for the purpose of supplementing the Aquatic Core Networks, which are comprised of Lotic and Lentic Core Areas and Aquatic Buffers: https://nalcc.databasin.org/datasets/3eeab562664b421ebc1b830151e8b4db

Description and Derivation

Analysis of resiliency and complexity were carried out on the level of stream network. All stream networks at least 3.2km long with watersheds greater than 100km2 were included. These constitute 78% of all stream kilometers in the Northeast and Mid-Atlantic regions.

Similar stream networks were compared within an appropriate context by subdividing the region according to two nested classifications: Freshwater Ecoregion (World Wildlife Fund, Abell et al., 2008),  patterns of native fish distribution resulting from large-scale geoclimatic processes and evolutionary history; and fish regions, defined by cluster analysis of native species occurrence data  from NatureServe.

Resiliency scores were calculated from physical properties (stream network length, number of size classes, number of gradients classes and number of temperature classes), and condition characteristics (risk of hydrologic alterations, natural cover in the floodplain, and amount of impervious surface in the watershed). Complex stream networks were distinguished from non-complex as those containing five or more size stream, river, and lake size classes.

The high and highest resiliency stream networks included in this dataset were identified as follows:

Highest Relative Resilience

1)      Scores for physical properties and condition characteristics were each >=0.5 SD (above average) compared with all functionally connected stream reaches assessed within their freshwater ecoregion or fish region, or

2)      The sum of the physical properties and condition scores was at least 1.5 SD above the mean and the lowest score was between -0.5 and 0.5 SD (within the range of the mean) within their freshwater ecoregion or fish region.

High Relative Resilience

1)      Scores for physical properties and condition characteristics were  each above the calculated mean (> 0 z-unit)  but one or both were less than 0.5 SD  within their freshwater ecoregion or fish region, or

2)      The sum of both scores was at least >1 SD above the mean and both the physical property and condition score were between -0.5 and 0.5 SD (within the range of the mean) for their freshwater ecoregion or fish region).

Stream networks were translated into watersheds by delineating the land draining into the stream and rivers of each functional network using the “watershed” command run on the NHD Plus Medium Resolution 30m flow direction grids distributed by USGS and TNC’s hydrology lines (which break at dams).

Known Issues and Uncertainties

As with any project carried out across such a large area, this product is subject to limitations. The results by themselves are not a prescription for on-the-ground action; users are encouraged to verify, with field visits and site-specific knowledge, the value of any areas identified in the project. Known issues and uncertainties include the following:

• Fish regions are based on lists of native species present within each HUC, which in turn depend on data from Natural Heritage program mapping. That data is suject to variation in the suite of species tracked within each jurisdiction. Additionally, there are gaps in geographic coverage both within and among states with variation in survey effort.

• Access to groundwater, would ideally be included in the set of measurable elements used characterize the resilience of freshwater systems, was excluded from this analysis due to data limitations at this scale.