A dam is a barrier constructed across a river, built primarily to store water in a reservoir for purposes like irrigation, power generation, or flood control. Wetland ecosystems, including marshes, swamps, and bogs, are areas saturated with water, either seasonally or permanently. These habitats are heavily reliant on the natural flow of the rivers they border and are sensitive to changes in water supply and quality. The construction and operation of dams fundamentally alter river systems, posing a significant threat to the health and function of connected wetland habitats.
Disrupting the Natural Flow Regime
The most immediate impact of a dam is the radical change it imposes on the river’s natural hydrological cycle—the historical pattern of water flow, volume, and timing. Wetlands are adapted to a natural hydrograph featuring distinct seasonal high-flow events and low-flow periods. Dam operations flatten this natural cycle, often eliminating the annual spring flood pulses necessary for wetland maintenance.
These seasonal flood pulses recharge groundwater stores, disperse plant seeds, and reconnect the main river channel to its floodplain wetlands. When a dam withholds this surge, downstream wetlands suffer from “water starvation,” leading to a reduction in their area and a shift in their ecological character. The reduction in the duration and frequency of high flow pulses contributes to a decline in riparian wetland area along affected river reaches.
The timing and volume of water delivery become controlled by human needs, such as power generation or irrigation schedules, rather than environmental requirements. This results in unnatural low-flow conditions during high-water seasons or erratic, non-seasonal releases that do not mimic a natural flood. Such altered flow regimes prevent wetlands from performing their functions, causing them to dry out and converting productive marshland into less diverse, terrestrial habitats.
Trapping Sediments and Essential Nutrients
Dams act as sediment traps, impounding the sand, silt, and organic matter that a river naturally carries downstream. This material is a fundamental component of the riverine ecosystem, providing two functions for downstream wetlands: elevation maintenance and nutrient supply. Without this regular deposit of fresh sediment, wetlands begin to subside due to natural compaction and decomposition, a situation exacerbated by sea-level rise in coastal areas.
The sediment carries essential nutrients, such as nitrogen and phosphorus, necessary to support the wetland food web and plant life. Retention of these materials behind the dam significantly reduces the nutrient load available to downstream ecosystems, altering the chemical composition of the water and soil. This sediment-starved water is often referred to as “hungry water” because it has a higher capacity for erosion.
This “hungry water” aggressively erodes the riverbed and banks downstream of the dam, incising the channel deeper into the landscape. This incision physically disconnects the river from its adjacent floodplain wetlands, making it difficult for water to spill over and recharge the wetlands during high-flow periods. Degradation of the riverbanks further destabilizes wetland borders, accelerating their loss and conversion to dry land.
Degradation of Water Quality
The deep, slow-moving water impounded behind a dam forms a reservoir that behaves more like a lake than a river, leading to thermal stratification. During warmer months, the water column separates into distinct layers: a warmer, oxygen-rich surface layer (epilimnion) and a colder, denser bottom layer (hypolimnion). Decaying organic matter in the bottom layer consumes available oxygen, often leading to hypoxic or anoxic (low or no oxygen) conditions.
When water is released from the dam’s lower outlets, the discharge is unnaturally cold, low in dissolved oxygen, and can be chemically altered—a process known as hypolimnetic release. This cold-water pollution stresses wetland organisms adapted to warmer, natural river temperatures, causing delays in fish spawning or forcing temperature-sensitive species to emigrate.
Anoxic conditions in the hypolimnion can cause chemical changes, such as the release of dissolved metals or hydrogen sulfide from the reservoir sediments. These chemical stressors, combined with low oxygen levels, harm downstream aquatic life and change the composition of macroinvertebrates and other organisms that form the base of the wetland food chain.
Consequences for Wetland Biodiversity
The collective changes in water flow, sediment delivery, and water quality result in consequences for the diversity of life within wetland ecosystems. A visible impact is the obstruction of migratory fish species, such as salmon and sturgeon, which are prevented from reaching their upstream spawning grounds. Even with fish passage structures like ladders, migration effectiveness is often reduced, leading to population declines and loss of genetic diversity.
The elimination of seasonal flooding directly impacts specialized wetland plant species that rely on these events for their life cycle. Many riparian plants time seed dispersal to coincide with the high-water period, ensuring seeds are spread across the newly inundated floodplain. Without this pulse, reproduction is inhibited, leading to a shift in vegetation composition as less-adapted species encroach on the drying wetland edges.
Overall, the altered conditions favor generalist species and non-native organisms that can tolerate the colder, lower-oxygen water or the new, stable reservoir habitat. The loss of specialized plants and the decline of migratory fish populations signal a broader ecosystem shift, converting productive, diverse wetlands into less complex habitats.