A Free-flowing Rivers Assessment (FRA) is an assessment to determine the connectivity status of rivers by taking into account both natural connectivity as well as fragmentation from infrastructure, such as dams, roads, urban areas, water use.
Figure 1 introduces important concepts, such as the elements of the hydrographic framework, the dimensions of river connectivity and the determination of the river status. The main result of a FRA is a connectivity status index (CSI) representing how well river stretches are connected in the up- and downstream, as well as other directions, given existing infrastructure. The CSI ranges from 0 to 100% on a sliding scale. The index is used in a subsequent step to classify rivers as free-flowing or not.
In the global assessment, we gathered a team of scientists and practitioners, and first developed an integrated definition of free-flowing rivers (FFR) (Figure 2; step 1) according to multiple aspects of connectivity. Next, we identified five major pressure factors (step 2) that influence river connectivity according to an extensive literature review, and collated data for each factor. These five pressure factors include: (a) river fragmentation; (b) flow regulation; (c) sediment trapping; (d) water consumption; (e) road construction; and (f) urbanization.
We calculated proxy indicators (see Figure 3) for each factor using data from available global remote sensing products, other data compilations, or numerical model outputs such as discharge simulations. We specifically chose indicators that we expect to have substantial influence on connectivity and can be generated using robust global data sets of sufficient quality and consistency between countries and regions. All indicators were calculated for every river reach of the global river network (step 3).
Guided by literature reviews and expert judgement, we iteratively adjusted the weighting of each pressure indicator in a set of scenarios and tested different thresholds to yield a best match between the resulting FFRs and a benchmarking dataset of reported FFRs compiled from literature resources and expert input.
The final selection of weights was applied to a multi-criteria average calculation (step 4) to derive the Connectivity Status Index (CSI) for every river reach (step 5). The CSI ranges from 0% to 100%, the latter indicating full connectivity. Only river reaches with a CSI of >95% were considered as having ‘good connectivity status’ while river reaches below 95% were classified as impacted (step 6). Finally, river reaches were aggregated into rivers, i.e., contiguous flow paths from the source to the river outlet. If a river is above the CSI threshold of 95% over its entire length it is declared to be a FFR. Otherwise, the river as a whole is declared not free-flowing, yet it can maintain a mix of stretches with ‘good connectivity status’ and stretches that are impacted.
Figure 4 further illustrates the different concepts used in an FRA. The baseline river network consists of individual ‘river reaches’ (1–32 in a), defined as line segments separated by confluences (black dots). River reaches can be aggregated into ‘rivers’ according to a ‘backbone’ ordering system, which classifies river reaches as the mainstem or a tributary of various higher orders (b). Following this system, the river network can be distinguished into distinct rivers (1–16 in c), defined as contiguous stretches of river reaches from source to outlet on the mainstem or from source to confluence with the next-order river. CSI values for individual river reaches, as calculated with our model (d). If a value is at or above the CSI threshold (95%), the river reach is declared to have good connectivity status; if it is below the threshold, it is declared to be impacted. If an entire river (as defined in c) has good connectivity status )see panel e), it is defined to be an FFR (blue). A river can be partly above the CSI threshold, and thus contiguous stretches can have good connectivity status (green).
For more detailed information on the FRA methodology, please also refer to:
Grill, G., B. Lehner, M. Thieme, B. Geenen, D. Tickner, F. Antonelli, S. Babu, P. Borrelli, L. Cheng, H. Crochetiere, H. Ehalt Macedo, R. Filgueiras, M. Goichot, J. Higgins, Z. Hogan, B. Lip, M. E. McClain, J. Meng, M. Mulligan, C. Nilsson, J. D. Olden, J. J. Opperman, P. Petry, C. Reidy Liermann, L. Saenz, S. Salinas-Rodriguez, P. Schelle, R. J. P. Schmitt, J. Snider, F. Tan, K. Tockner, P. H. Valdujo, A. van Soesbergen and C. Zarfl (2019). "Mapping the world's free-flowing rivers." Nature 569(7755): 215-221.
