Rules of thumb for prioritizing barrier removals emerging from comprehensive analysis of great lakes tributaries
In most river networks, both dams and road crossings fragment habitat and impede fish migrations. Because removing these barriers is a costly process and resources are limited, tools are needed to guide prioritization of potential projects. We developed a mathematical optimization model for barrier removals in the Great Lakes basin and an associated online decision support tool (DST) designed for agency and NGO staff. The prioritization model weighs estimated cost of replacing each dam or culvert against length of upstream channel gained, and finds the portfolio of projects that maximizes habitat gains for a given budget. We parameterized this model using a recently-developed dataset describing the location, passability and estimated removal costs of hundreds of thousands of barriers. Across a broad range of budgets, we tested whether landscape variables can predict the likelihood a particular barrier is recommended for removal. These patterns could offer useful rules of thumb for decision-making even in regions where mapping and optimization modeling is impractical.
We observed striking differences in the potential costs and benefits of barrier removal projects across the Great Lakes basin. For example, projected removal costs varied almost 200-fold and even greater disparity existed in the total tributary length opened upstream of each barrier. The distributions of both costs and benefits exhibited high positive skewness with most projects being relatively inexpensive (< $300 k) and restoring access to a modest amount of tributary habitat (< 3.75 km). We found that the dramatic differences in upstream habitat among barriers were the primary driver of which barrier removal projects were included in the optimal project portfolios and that both dam and inexpensive road crossing projects were critical components of the most cost-effective strategies. Priority barrier removal projects were geographically widespread, indicating opportunities for cost-effective restoration of tributary connectivity across the entire Great Lakes basin. Similarly, priority barrier removal projects existed on a range of tributary sizes, from mainstem tributaries to smaller headwaters. We will discuss how our rules-of-thumb might be used to prioritize on-the-ground evaluation of barrier removal projects, and how these rules-of-thumb can support screening of barrier removal projects in cases where the data needed to parameterize optimization models are unavailable.