Prioritizing restoration of structural connectivity in rivers: a graph based approach

Longitudinal connectivity is considered a key issue in river management, as it shapes ecological processes from single organisms to populations and ecosystems. Recently, it was shown that network analysis based on spatial graphs has promising applications as a tool for the assessment of connectivity in riverine systems. In this study we used a graph theory approach to identify which barriers most impacted the structural connectivity of a river basin and which connections should preferably be restored or enhanced in order to effectively improve the overall connectivity. An innovative aspect of the proposed methodology is the consideration of the cumulative non-additive impacts produced by barriers, which are especially relevant to organisms of high mobility such as fish. The portuguese river Tagus basin was used as a case study. The cumulative effect of barriers was studied using two approaches: (1) an historical approach in which the impact of barriers was assessed sequentially following the historical succession of construction; (2) a "backward" approach in which barriers were sequentially removed according to their impact. The overall structural connectivity of the river basin decreased to about 50 % of its original value after the major dams were constructed. Results show that it would be necessary to rehabilitate 11 connections in order to increase the overall structural connectivity to 90 % of its original value. This work proposes a novel and straightforward approach to prioritize rehabilitation actions in river systems, providing a promising tool for decision-makers.