Structures that block movement of fish through river networks are built to serve a variety of societal needs, including transportation, hydroelectric power, and exclusion of exotic species. Due to their abundance, road crossings and dams reduce the amount of habitat available to fish that migrate from the sea or lakes into rivers to breed. The benefits to fish of removing any particular barrier depends on its location within the river network, its passability to fish, and the relative position of other barriers within the network. Balancing the trade‐offs between ecological and societal values makes choosing among potential removal projects difficult. To facilitate prioritization of barrier removals, we developed an online decision support tool (DST) with three functions: (1) view existing barriers at various spatial scales; (2) modify information about barriers, including removal costs; and (3) run optimization models to identify portfolios of removals that provide the greatest amount of habitat access for a given budget. A survey of available DSTs addressing barrier removal prioritization indicates that barrier visualization is becoming widespread but few tools allow dynamic calculation of connectivity metrics, scenario analysis, or optimization. Having these additional functions, our DST enables organizations to develop barrier removal priorities based on cost‐effectiveness in restoring aquatic connectivity.

Controlling invasive species is critical for conservation but can have unintended consequences for native species and divert resources away from other efforts. This dilemma occurs on a grand scale in the North American Great Lakes, where dams and culverts block tributary access to habitat of desirable fish species and are a lynchpin of long-standing efforts to limit ecological damage inflicted by the invasive, parasitic sea lamprey (Petromyzon marinus). Habitat restoration and sea-lamprey control create conflicting goals for managing aging infrastructure. We used optimization to minimize opportunity costs of habitat gains for 37 desirable migratory fishes that arose from restricting sea lamprey access (0-25% increase) when selecting barriers for removal under a limited budget (US$1-105 million). Imposing limits on sea lamprey habitat reduced gains in tributary access for desirable species by 15-50% relative to an unconstrained scenario. Additional investment to offset the effect of limiting sea-lamprey access resulted in high opportunity costs for 30 of 37 species (e.g., an additional US$20-80 million for lake sturgeon [Acipenser fulvescens]) and often required ≥5% increase in sea-lamprey access to identify barrier-removal solutions adhering to the budget and limiting access. Narrowly distributed species exhibited the highest opportunity costs but benefited more at less cost when small increases in sea-lamprey access were allowed. Our results illustrate the value of optimization in limiting opportunity costs when balancing invasion control against restoration benefits for diverse desirable species. Such trade-off analyses are essential to the restoration of connectivity within fragmented rivers without unleashing invaders.

Dams, road crossings, and water withdrawals extensively fragment rivers, and watersheds often contain hundreds or thousands of barriers, some of which no longer meet societal purposes. Accordingly, both conservationists and infrastructure managers are faced with the challenge of prioritizing barriers for repair, replacement, or removal. Candidate projects have been prioritized with dozens of methods, which span a wide range of spatial scales, data and analytical requirements, mathematical complexity, and capacity to reconcile multiple perspectives and objectives. We briefly review barrier prioritization methods from the perspective of a policy maker or manager who must balance realities of stochastic opportunities, conflicting priorities, and risk of infrastructure failure. After outlining common motivations for barrier prioritization, we present a menu of techniques ranging from large-scale, quantitative assessments to reactive, local response to failures. By clarifying the appropriate domain for each approach, this review informs the selection of prioritization methods for restoring riverine connectivity. K E Y W O R D S aquatic organism passage, dam removal, portfolio analysis, spatial prioritization, watershed planning 1 | INTRODUCTION Dams, road crossings, and other built infrastructure disrupt river and stream ecosystems yet serve a variety of important societal purposes including management of flood risk, provision of reliable water supplies, and transportation of people and goods. Alleviating ecological problems caused by infrastructure represents a critical conservation challenge. Specifically, these structures reduce aquatic connectivity, thereby limiting the ability of aquatic animals to reach important breeding, refuge, and feeding habitats; disrupting the transport of sediment; and altering river hydrodynamics (Pringle, 2001). The scale of infrastructure impacts on aquatic ecosystems is tremendous (e.g., Barbarossa et al., 2020; Vörösmarty et al., 2010). Although comprehensive geospatial databases do not exist, tens of thousands of large dams (Bellmore et al., 2016) and millions of road-stream crossings (Januchowski-Hartley et al., 2013

Conservation planning aims to optimize outcomes for select species or ecosystems by directing resources toward high‐return sites. The possibility that local benefits might be increased by directing resources beyond the focal area is rarely considered. We present a case study of restoring river connectivity for migratory fish of the Great Lakes Basin by removing dams and road crossings within municipal jurisdictions versus their broader watersheds. We found that greater river connectivity could often be achieved by considering both intra‐jurisdictional and extra‐jurisdictional barriers. Focusing on jurisdictional barriers alone generally forfeited <20% (median = 0%) of habitat gains for those who value solely habitat gains within the jurisdiction, but >75% (median = 100%) for planners who value larger‐scale habitat gains. Similarly, cost savings tended to be between −50% and +50%, but in some cases were very negative. Our study underscores the local‐scale benefits of broadening restoration investments, especially for decision makers of the Great Lakes Basin and contributes to a discussion of appropriate and efficient scales of conservation planning. Copyright © 2017 John Wiley & Sons, Ltd.

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