Bidirectional connectivity in rivers and implications for watershed stability and management

River networks are connected in both upstream and downstream directions on large spatial scales by movement of water, materials, and animals. Here I examine the implications of these linkages for the stability, productivity, and management of watersheds and their migratory fishes. I use simple simulations of watershed alteration to illustrate that degradation can erode the productivity and stability of both upstream and downstream fisheries. Through analysis of an existing global dataset on rivers, I found that larger rivers tend to be more fragmented than smaller rivers. I offer three challenges and opportunities for the future management of watersheds. First, given that human impacts can spread up and down rivers, there is a need to align the scales of impact assessments with the natural scale of river systems. Second, free-flowing rivers naturally dampen variability; thus, the conservation of connectivity, habitat, and biodiversity represents a key opportunity to sustain the processes that confer stability. Third, watersheds represent natural units of social-ecological systems; watershed governance would facilitate reciprocal feedbacks between people and ecosystems and enable more social-ecological resilience.