Surreal illustration of a dam with a flawed fish passage system, highlighting the clash between engineering and ecology.

When Dams Don't Deliver: The Hidden Crisis of Fish Passage Systems

Elliot Brynn in Climate & Environment June 2025 • 5 min read.

"How ecohydraulic oversights are impacting fish populations in irrigation systems—and what we can do about it."

Dams are vital components of water management infrastructure worldwide, providing essential water supply for agriculture, industry, and domestic use. However, their construction often comes at a significant environmental cost, particularly concerning the disruption of natural river ecosystems. One of the most critical impacts is the obstruction of fish migration routes, which can lead to decreased biodiversity, reduced fish populations, and economic losses for communities dependent on fisheries.

In South America, the construction of dams has increased to meet growing water demands. While these projects aim to enhance water security, they frequently lack adequate consideration for fish passage. The consequences are far-reaching, affecting not only fish species but also the overall health and resilience of river systems. A recent study focusing on the Route 28 Dam in the Pilcomayo River basin sheds light on the challenges of designing effective fish passage systems in the region.

The Route 28 Dam, located in Formosa Province, Argentina, exemplifies the complexities of balancing water resource management and ecological preservation. Built to ensure year-round road crossing and provide water for irrigation, the dam has the potential to block fish movements between the La Estrella marsh and the Pilcomayo River. This obstruction can strand fish downstream, leading to high mortality rates during low water periods. The central question is whether the existing fish passage systems, specifically pool and weir ladders, are effective for key migratory species like the sábalo (Prochilodus lineatus).

The Engineering vs. Ecology Mismatch

Surreal illustration of a dam with a flawed fish passage system, highlighting the clash between engineering and ecology.

The study's findings reveal a significant mismatch between engineering designs and the ecohydraulic requirements of local fish species. The Route 28 Dam's spillway, designed to release excess water, poses a considerable barrier to fish attempting to migrate upstream. The research indicated that only fish longer than 39 cm could potentially ascend the spillway chute, and even then, only when water levels on the spillway crest exceed 0.4 meters. This limitation excludes a significant portion of the sábalo population, which typically ranges from 26 to 45 cm.

Further compounding the problem is the design of the dissipation pool at the spillway's base. The pool, intended to reduce the force of the water flow, does not meet the minimum depth criteria required for fish to accelerate to sufficient velocity to jump to the spillway crest. This design flaw effectively prevents fish from leaping over the dam, regardless of their size or swimming ability.

Spillway design restricts passage to larger fish during high water levels.
Inadequate dissipation pool depth prevents effective leaping.
Fish ladders suffer from design flaws, limiting their effectiveness.
Attraction flows are insufficient to guide fish to ladder entrances.

The fish ladders, designed as a series of pool and weir systems, also suffer from critical shortcomings. The number of pools is insufficient, and the dimensions and designs of some pools deviate from accepted standards. The volumetric dissipation power in the upper pool of each fish ladder is too low, failing to provide adequate resting areas for fish. Moreover, the attraction flows—the water flow intended to lure fish into the ladder entrances—are insufficient relative to the total spillway discharge. This means that fish are unlikely to find the entrances, especially given the turbulent conditions created by the spillway.

Rethinking Dam Design: A Call for Ecohydraulic Integration

The failures observed at the Route 28 Dam highlight the urgent need to move beyond "salmon-centric" designs and embrace ecohydraulic principles tailored to the unique characteristics of neotropical fish species and their environments. Future dam projects must consider the hydrological variability and bioecological factors specific to pulsatile systems like La Estrella marsh. This requires a holistic approach that integrates engineering, ecology, and local knowledge to create effective and sustainable solutions.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1590/s1679-62252012000400007, Alternate LINK

Title: Fish Passage System In An Irrigation Dam (Pilcomayo River Basin ): When Engineering Designs Do Not Match Ecohydraulic Criteria

Subject: Aquatic Science

Journal: Neotropical Ichthyology

Publisher: FapUNIFESP (SciELO)

Authors: Claudio R. M. Baigún, John M. Nestler, Priscilla Minotti, Norberto Oldani

Published: 2012-10-01

Everything You Need To Know

What are the primary environmental concerns associated with dam construction, specifically regarding fish populations?

Dams are constructed to ensure water supply for agriculture, industry, and domestic use. However, they can disrupt fish migration routes, leading to decreased biodiversity, reduced fish populations, and economic losses. For example, the Route 28 Dam, aimed to enhance water security, lacks adequate consideration for fish passage, affecting the health of river systems.

What is the purpose of the Route 28 Dam, and what are its potential impacts on fish migration in the Pilcomayo River basin?

The Route 28 Dam in Formosa Province, Argentina, aims to ensure year-round road crossing and provide water for irrigation. However, the dam has the potential to block fish movements between the La Estrella marsh and the Pilcomayo River, potentially stranding fish downstream and leading to high mortality rates during low water periods. The effectiveness of its fish passage systems, like pool and weir ladders, is questionable for key migratory species such as the sábalo (Prochilodus lineatus).

What are the key findings of the study on the Route 28 Dam regarding the effectiveness of its fish passage systems?

The study found a mismatch between engineering designs and the ecohydraulic requirements of local fish species at Route 28 Dam. The spillway design restricts passage to larger fish only during high water levels. The inadequate dissipation pool depth prevents effective leaping, while the fish ladders suffer from design flaws, limiting their effectiveness. The attraction flows are also insufficient to guide fish to ladder entrances.

How does the spillway design at the Route 28 Dam specifically restrict fish passage, and what limitations does it impose on species like the sábalo?

The spillway design at the Route 28 Dam restricts fish passage because only fish longer than 39 cm can potentially ascend it, and even then, only when water levels on the spillway crest exceed 0.4 meters. This excludes a significant portion of the sábalo population. Additionally, the dissipation pool at the spillway's base does not meet the minimum depth criteria required for fish to accelerate to sufficient velocity to jump to the spillway crest, hindering their ability to leap over the dam.

What changes are needed in dam design to better accommodate fish migration, based on the ecohydraulic failures identified at the Route 28 Dam?

Addressing the failures observed at the Route 28 Dam requires moving beyond "salmon-centric" designs and embracing ecohydraulic principles tailored to the unique characteristics of neotropical fish species and their environments. Future dam projects must consider the hydrological variability and bioecological factors specific to pulsatile systems like La Estrella marsh. This holistic approach integrates engineering, ecology, and local knowledge to create effective and sustainable solutions.