Road Stream Crossing Upgrade Effects, Northeast U.S.

This dataset is one of a suite of products from the Nature’s Network project (naturesnetwork.org). Nature’s Network is a collaborative effort to identify shared priorities for conservation in the Northeast, considering the value of fish and wildlife species and the natural areas they inhabit. This dataset represents the relative potential to improve local aquatic connectivity by upgrading road-stream crossings. The model incorporates survey data from the North Atlantic Aquatic Connectivity Collaborative (NAACC). To view the current NAACC database go to https://streamcontinuity.org/database.htm. The Road Stream Crossing Upgrade Effects dataset and other datasets that augment or complement aquatic connectivity are available in the Nature’s Network gallery: https://nalcc.databasin.org/galleries/8f4dfe780c444634a45ee4acc930a055.

In summary, this dataset summarizes opportunities to restore local aquatic connectedness by upgrading road-stream crossings. Aquatic connectedness represents the amount of ecological flow (e.g., movement of aquatic organisms) upstream and downstream from a focal cell, weighted by geographical distance and ecological similarity.

Intended Uses

This product is intended to identify locations where barriers to aquatic passage may exist, in the form of road-stream crossings, and to serve as one tool to inform decisions on investments to improve aquatic passage. Note that in the scoring system used in this product, the improvement of any one road-stream crossing typically does not have a large effect on aquatic connectedness and consequently most crossings are scored as “Very Low Effect.” Crossings within this category can still be compared by looking at their “Effect” scores or their ranks. Currently, the effect scores and categories are most relevant for scenarios considering resident rather than diadromous species because the aquatic connectedness metric used in scoring does not take into account the needs of diadromous fish to migrate over long distances.

You might explore this dataset in combination with the following products:

• Lotic core areas (a part of Aquatic Core Networks) to identify rivers and streams considered to be of high conservation priority taking into account ecological condition and fish habitat.

• Brook Trout Probability of Occurrence, to identify the likelihood that Brook Trout occur in the vicinity of road stream crossings of interest.

• The Aquatic Barrier Prioritization Tool (http://maps.freshwaternetwork.org/northeast/; not currently part of Nature’s Network) to find other information relevant to road stream crossing decisions, including information about potential benefits to diadromous fish.

Description and Derivation

The culvert upgrade metric measures the improvement in aquatic connectedness from upgrading a road-stream crossing from a culvert with its estimated degree of passability for aquatic organisms to a bridge with minimal impediment to ecological flows. The result is a dataset with a point location for each estimated road stream crossing and a suite of attributes about the crossing and an estimate of the effect of upgrading the crossing to a bridge based on the difference in aquatic connectedness. The effects on aquatic connectedness are weighted more heavily where areas are already in good ecological condition (based on an Index of Ecological Integrity). Scores are therefore lower where conditions are already so degraded that an upgrade would not appreciably improve local ecosystem conditions.

The following technical documentation provides more information about this dataset: http://jamba.provost.ads.umass.edu/web/lcc/DSL_documentation_criticalLinkages.pdf.

Known Issues and Uncertainties

As with any project carried out across such a large area, this dataset 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, any road stream crossings and associated aquatic habitat identified in the project. Known issues and uncertainties include the following:

• The results do not incorporate important social, economic, or feasibility factors that are important in determining cost-benefit tradeoffs of any particular culvert upgrade.

• Aquatic barrier scores and the subsequent aquatic connectedness scores, and thus the culvert upgrade and dam removal scores, are derived from a model, and thus subject to the limitations of any model due to incomplete and imperfect data, and a limited understanding of the phenomenon being represented. The GIS data on road-stream crossings are imperfect; some crossings may be missing and some mapped crossings may not exist in the real world. Moreover, the vast major of road-stream crossings have not been surveyed in the field, and their predicted aquatic barrier scores are based on a very simple model derived from GIS data. Thus, culvert upgrades and dam removals should be used and interpreted with a healthy degree of caution and an appreciation for the limits of the available data.

• In the scoring system used in this product, the improvement of any one road-stream crossing typically does not have a large effect on aquatic connectedness and consequently most crossings are scored as “Very Low Effect.” Crossings within this category can still be compared by looking at their effect scores or their ranks.

• Currently, the effect scores and categories are most relevant for scenarios considering resident rather than diadromous species because the aquatic connectedness metric used in scoring does not take into account the needs of diadromous fish to migrate over long distances.

• The effect scores do not account for the potential for multiple upgrades of nearby crossings, which could have greater improvements to aquatic connectivity than a single upgrade.

• For the road-stream crossings assessed in the field, we use an algorithm developed by the River and Stream Continuity Partnership (2010, www.streamcontinuity.org) for scoring crossing structures according to the degree of obstruction they pose to aquatic organisms. Of course, as with any such algorithm, it cannot deal effectively with the myriad species-specific constraints on passability that affect the entire aquatic community. Thus, the score must be viewed as a generalized index on aquatic passability and cannot be used to infer passability for any single species.

• While the dataset has a wide variety of potential uses, its primary utility is to aid in the prioritization of road-stream crossings for culvert upgrades. However, because of the considerations discussed above, it is probably best used at the watershed or regional scale for broad-scale strategic planning, e.g., identifying subbasins where significant improvements in local connectivity might be achieved through one or more culvert upgrades or dam removals, or prioritizing field surveys of road-stream crossings to improve aquatic barrier scores.