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Abstract Large dams are a leading cause of river ecosystem degradation. Although dams have cumulative effects as water flows downstream in a river network, most flow alteration research has focused on local impacts of single dams. Here we examined the highly regulated Colorado River Basin (CRB) to understand how flow alteration propagates in river networks, as influenced by the location and characteristics of dams as well as the structure of the river network—including the presence of tributaries. We used a spatial Markov network model informed by 117 upstream‐downstream pairs of monthly flow series (2003–2017) to estimate flow alteration from 84 intermediate‐to‐large dams representing >83% of the total storage in the CRB. Using Least Absolute Shrinkage and Selection Operator regression, we then investigated how flow alteration was influenced by local dam properties (e.g., purpose, storage capacity) and network‐level attributes (e.g., position, upstream cumulative storage). Flow alteration was highly variable across the network, but tended to accumulate downstream and remained high in the main stem. Dam impacts were explained by network‐level attributes (63%) more than by local dam properties (37%), underscoring the need to consider network context when assessing dam impacts. High‐impact dams were often located in sub‐watersheds with high levels of native fish biodiversity, fish imperilment, or species requiring seasonal flows that are no longer present. These three biodiversity dimensions, as well as the amount of dam‐free downstream habitat, indicate potential to restore river ecosystems via controlled flow releases. Our methods are transferrable and could guide screening for dam reoperation in other highly regulated basins.
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Fish assemblage response to removal of a low‐head dam in the lower reach of a tropical island river
Abstract Dams are often removed from rivers to restore habitat connectivity for biota such as fish. Removal of inland dams is well studied in temperate mainland rivers but this approach has been little studied in fish assemblages in islands, tropic systems, or for dams near the mouth of the river. In Puerto Rico, one of the most intensively dammed territories in the world, all native river fishes migrate between fresh water and the sea, and previous work shows that these movements are impeded or blocked by dams.Fish assemblages were compared before and after removal of the Cambalache dam, a porous, low‐head structure near the mouth of the Río Grande de Arecibo, as well as in two other rivers in western Puerto Rico, one with a similarly sized and positioned dam, and one reference river without artificial barriers. Fish were sampled using backpack electrofishing on 39 occasions during 2017–2019, including seven samples collected after removal of the Cambalache dam, at four to six sites per river.Fish assemblages upstream from dams were poorer in species, and species richness showed a marginal tendency (p = 0.0515) to increase upstream of the Cambalache dam 3 months after its removal. The two small lowland dams studied herein limited the upstream extent of marine species, which recolonised upstream sites of the Río Grande de Arecibo after removal of the Cambalache dam. An estimate of relative density (catch per unit effort) of common native freshwater species was higher above these two dams, and decreased at upstream sites after removal of the Cambalache dam. The estimated relative density of a native freshwater species that is of conservation concern, the American eel (Anguilla rostrata), was reduced above dams, and increased upstream of the former Cambalache dam after its removal.In extensive surveys conducted previously in Puerto Rico, sampling was concentrated higher in the catchment, and native fishes were more common and abundant below than above dams. The present work was conducted near the river mouth, and opposite results were observed. These contrasting results suggest that the effects of dams (or dam removal) on fish assemblages vary along the river gradient, although data from other systems are needed to confirm this.The present results suggest low‐head dam removal to be a viable method of restoring connectivity in fish assemblages in lower reaches of rivers in Puerto Rico and, potentially, other tropical islands. Removal of dams near the mouth of the river appears to be of particular benefit to marine fish species that use lower river reaches.
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- Award ID(s):
- 1845021
- PAR ID:
- 10367522
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Freshwater Biology
- Volume:
- 67
- Issue:
- 5
- ISSN:
- 0046-5070
- Page Range / eLocation ID:
- p. 926-937
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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