When Dams Don't Deliver: The Hidden Crisis of Fish Passage Systems
"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 Scale of the Fish Passage Challenge
Fish passage systems—including ladders, elevators, and bypasses—have become standard features at many dams worldwide, designed to reduce the impact of barriers on fish migration. Yet designing structures that perform well biologically, rather than simply meeting hydraulic criteria, remains a persistent engineering challenge. The two largest hydropower production facilities in the US, both located in the Columbia River system, still lack fish passage capabilities entirely. The success of any given system depends heavily on the biology and behavior of the target species, as well as hydrologic conditions both upstream and downstream of the project.
Design Flaws Undermine Fish Passage Effectiveness
Research on the Route 28 Dam in the Pilcomayo River basin reveals how fish passage systems can fail when engineering designs do not match ecohydraulic criteria. The system installed at this irrigation dam does not follow the general principles advocated in the fish passage literature, failing to function under varying hydrological conditions and lacking suitable attraction flows. Analysis shows that only fish longer than 39 centimeters can ascend the spillway under specific conditions, highlighting significant design inadequacies. These failures trace back to a "salmon-centric" conceptual approach applied inappropriately to a system dominated by different species.
Decades of Adaptation in Fish Passage Infrastructure
Mud Mountain Dam, originally built in 1948, required a $112 million fish passage upgrade because its existing facility was never intended to handle the migration volumes that eventually arrived during peak seasons. At the Merwin Dam on the Lewis River, a fish ladder and sorting facility routes collected fish either to a local hatchery or releases them upstream of Swift Dam to spawn naturally. These cases illustrate how fish passage infrastructure built in earlier decades often proves inadequate as migration patterns and volumes evolve over time.
The Engineering vs. Ecology Mismatch
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.
- 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.
Advancing Fish Passage Science Through New Methodologies
A growing body of literature confirms that numerous fish passage structures have been designed and implemented to mitigate the negative effects of dams on fish survival and population status. Research into trap-and-haul facilities shows that attraction flow is primarily influenced by river discharge conditions and fish migration period, while fishway flow is determined by the hydraulic characteristics of the fishway design. Computational fluid dynamics applications in fishway research have expanded, though systematic reviews highlight the need for greater model validation and three-dimensional modeling to improve fish passage solutions. These converging lines of research underscore the interdisciplinary nature of modern fishway engineering.
When Fish Passage Systems Fall Short
The Route 28 Dam case study demonstrates how fish passage systems can fail when designers apply a "salmon-centric" concept to ecosystems with different species compositions. At Oregon's Green Peter Dam, a mothballed fish passage system raised concerns that confining fish in close quarters could spread disease, with project managers noting that if one fish gets sick, the others might follow. However, initial experiments showed most fish survived their 48-hour captivity, suggesting trap-and-haul approaches may still hold promise when properly managed. These contrasting outcomes underscore that fish passage failures often stem from design decisions that ignore local ecological realities.
Balancing Tradeoffs Across Fish Passage Approaches
Fish passage solutions vary widely in their effectiveness depending on dam characteristics, target species, and local environmental conditions. Simple structural solutions such as rock ramps may suit smaller barriers, while large storage reservoirs often require more complex engineered systems. No single approach has emerged as universally effective, and the tradeoffs between cost, maintenance demands, and biological performance remain subjects of ongoing debate. Context-specific evaluation is essential, as solutions that perform well in one river system may fail in another.
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.
Expert Consensus on the State of Fish Passage
A comprehensive review authored by 17 experts from diverse fish passage research fields concludes that fishway science, engineering, and practice remain imperfect despite decades of effort devoted to developing, constructing, and refining passage facilities. The experts acknowledge that much work has gone into enabling target species to pass barriers on fluvial systems, yet significant gaps persist between design intentions and biological outcomes. This consensus reflects a growing recognition within the field that incremental improvements to existing approaches may not be sufficient to address the scale of the challenge.
Innovations Pointing Toward Better Fish Passage
The Fishheart system, an innovative fish passage technology, was scheduled for installation at the Lake Wetherell outlet regulator during the 2024/25 summer season, with expectations of significantly improving fish passage between the Northern and Southern Basin. Consulting firms Kleinschmidt Associates and Anchor QEA have partnered to lead development of fish passage systems at Chief Joseph and Grand Coulee dams, two of the largest dams in the Pacific Northwest. In Oregon, improvements to fish passage at the Leaburg run-of-river hydroelectric project represent progress, though experts note it is a fundamentally different challenge than passage at large federal storage reservoirs like Detroit or Green Peter.
Systemic Gaps in Fish Passage Performance
Data from the Yacyretá dam reveals alarmingly low fish passage efficiency of less than 2% in fish elevators, with about 85% of captured fish belonging to only three nonmigratory species—suggesting these systems may be collecting the wrong fish entirely. Meanwhile, advances in fish-friendly turbine design offer a complementary approach, with evidence suggesting such turbines can reduce river mortalities by up to 80% through streamlined blades and adjustable flow controls. Together, these findings highlight that both passage infrastructure and turbine technology must improve to meaningfully protect aquatic ecosystems.
Real-World Consequences of Passage Failures
When fish passage systems fail to deliver, the consequences extend beyond ecology to communities that depend on healthy fish populations for cultural identity and livelihoods. Dam operators face increasing legal and regulatory pressure to improve passage, as evidenced by ongoing litigation over salmon survival in the Pacific Northwest. The gap between engineering intent and on-the-ground outcomes ultimately affects not just fish, but the broader social and economic systems intertwined with river health.