Hydrologic connectivity is critical to the structure, function, and dynamic process of river ecosystems. Dams, road crossings, and water diversions impact connectivity by altering flow regimes, behavioral cues, local geomorphology, and nutrient cycling. This longitudinal fragmentation of river ecosystems also increases genetic and reproductive isolation of aquatic biota such as migratory fishes. The cumulative effects on fish passage of many structures along a river are often substantial, even when individual barriers have negligible impact. Habitat connectivity can be improved through dam removal or other means of fish passage improvement (e.g., ladders, bypasses, culvert improvement). Environmental managers require techniques for comparing alternative fish passage restoration actions at alternative or multiple locations. Herein, we examined a graph-theoretic algorithm for assessing upstream habitat connectivity to investigate both basic and applied fish passage connectivity problems. First, we used hypothetical watershed configurations to assess general alterations to upstream fish passage connectivity with changes in watershed network topology (e.g., linear vs. highly dendritic) and the quantity, location, and passability of each barrier. Our hypothetical network modeling indicates that locations of dams with limited passage efficiency near the watershed outlet create a strong fragmentation signal but are not individually sufficient to disconnect the system. Furthermore, there exists a threshold in the number of dams beyond which connectivity declines precipitously, regardless of watershed topology and dam configuration. Watersheds with highly branched configurations are shown to be less susceptible to disconnection as measured by this metric. Second, we applied the model to prioritize barrier improvement in the mainstem of the Truckee River, Nevada, USA. The Truckee River application demonstrates the ability of the algorithm to address conditions common in fish passage projects including incomplete data, parameter uncertainty, and rapid application. This study demonstrates the utility of a graph-theoretic approach for assessing fish passage connectivity in dendritic river networks assuming full basin utilization for a given species, guild, or community of concern.

One study outcome was the development of a Comprehensive Restoration Plan (CRP) that serves as a master plan and blueprint for future restoration in the HRE. The CRP's goal is to develop a mosaic of habitats that provide the public with renewed and increased benefits from the estuary. In addition, the CRP provides the framework for an estuary-wide ecological restoration program by utilizing restoration targets-Target Ecosystem Characteristics (TECs)-developed by the region's stakeholders. One TEC focuses on restoring tributary environments and reconnecting rivers to coastal environments that benefit impacted or imperiled migratory fishes (e.g., Alewife, blueback herring, Striped bass, American shad, and American eel). This technical note describes a procedure developed to prioritize removal of major migratory barriers, specifically dams. These methods are demonstrated in one of eight planning regions, the Harlem River, East River, and Western Long Island Sound Planning Region, where they were applied to prioritize potential barriers for removal over a range of costs. The prioritization scheme is based on four primary components: habitat quantity upstream of a dam, habitat quality upstream of a dam, the effects of multiple dams in sequence in the context of diadromous fish (i.e., if a fish cannot pass the most downstream dam, then upstream dam removal provides no benefits), and a rapid, screening-level relative cost estimate. This technique is then applied to examine 49 potential dam removal sites. A combinatorial algorithm was applied to develop plans with more than 489,000 combinations of removal sites (e.g., remove barrier-A, barrier-B, neither, or both). From this analysis, 49 proposed sites were screened and refined to a recommended plan containing 12 sites, which provides 66% of the total potential habitat gain at 19% of the relative cost. The advantages and challenges of barrier prioritization are then discussed more broadly with an emphasis on efficiencies that can arise as a result of spatial prioritization methods.