Wetlands are unique ecosystems existing at the dynamic boundary between dry land and open water bodies. They are defined by the prolonged presence of water that saturates the soil, fostering specialized vegetation adapted to these waterlogged conditions. Found extensively on floodplains alongside rivers and streams, wetlands function as a natural buffer system connected to the surrounding watershed. Their physical relationship to river systems means any change to the wetland landscape has immediate consequences for the health and stability of adjacent waterways.
Alterations to Water Flow and Storage
The removal of wetlands eliminates a massive natural storage mechanism, significantly disrupting flow patterns in rivers and streams. Wetlands, particularly those on floodplains, act like sponges, absorbing and holding vast quantities of water during heavy rainfall or rapid snowmelt. This function, known as flood attenuation, reduces the volume and speed of water entering the main river channel, lowering the peak flow and minimizing severe downstream flooding.
When this storage capacity is lost, precipitation rapidly converts into surface runoff, causing faster and higher flood crests in the receiving river or stream. This increases the frequency and intensity of high-flow events, exerting damaging physical stress on the downstream ecosystem. Conversely, the slow-release function of wetlands is also lost, which is problematic during dry seasons.
Wetlands slowly meter out stored water to streams long after a rain event has passed, a process crucial for maintaining a river’s “base flow.” This steady input of water sustains the stream during prolonged dry periods, ensuring continuous aquatic habitat. Without this sustained release, rivers and streams experience more frequent and severe low-flow conditions, sometimes drying up entirely and fundamentally altering the stream’s year-round ecology.
Decline in Water Purity
Wetlands perform a sophisticated chemical and physical purification service, acting as efficient natural filters for the water that passes through them before it enters the river system. The dense network of wetland vegetation and the slow movement of water allow suspended solids, such as fine soil particles and organic matter, to settle out through sedimentation. This physical trapping significantly reduces the turbidity of the water delivered to the stream.
The loss of wetlands removes the necessary conditions for biological nutrient processing, which is important for controlling nitrogen pollution. Wetland soils are often anaerobic, or oxygen-poor, facilitating denitrification carried out by specialized bacteria. These microbes convert dissolved nitrates, often from agricultural runoff, into harmless nitrogen gas released into the atmosphere, effectively removing the pollutant.
Phosphorus, another major nutrient pollutant, is primarily removed through chemical processes, binding or adsorption to the metal ions and sediments within the wetland soil. When wetlands are removed, these excess nutrients flow directly into rivers and streams, leading to eutrophication. This nutrient loading fuels excessive algal growth, which then dies and decomposes, depleting dissolved oxygen in the water and creating hypoxic zones harmful to aquatic life.
Increased Stream Channel Erosion
Wetland vegetation plays a direct physical role in stabilizing the soil along the water’s edge; its removal leaves stream banks vulnerable to the erosive power of high-velocity flows. The roots of wetland plants form a dense, stabilizing matrix that binds soil particles together, increasing the bank’s resistance to scour. The loss of this structural support means banks are easily undercut and washed away, especially during the higher peak flows resulting from the loss of flood attenuation.
This increased hydraulic energy and lack of stabilization lead to accelerated lateral erosion, causing the stream channel to widen, and vertical erosion, causing the channel to deepen or become “incised.” The widening and deepening introduces massive quantities of fine sediment into the river system, fundamentally altering the physical structure of the riverbed. This excess sediment, no longer trapped by the upstream wetland, degrades the stream’s physical habitat and water quality.
Loss of Aquatic Habitats
The cumulative impact of altered flow, increased sedimentation, and nutrient loading degrades the river’s biological communities. The influx of fine sediment from eroding banks smothers the coarse gravel beds required by many native fish species for spawning. This sedimentation fills the interstitial spaces between rocks, eliminating the complex microhabitats essential for aquatic macroinvertebrates, which form the base of the stream’s food web.
Furthermore, higher flows cause increased stream power, which can physically displace and scour aquatic organisms that are not adapted to such turbulent conditions. The loss of the wetland’s cooling effect, often due to riparian vegetation removal, causes increased water temperatures in the stream. Combined with oxygen depletion from decomposing algal blooms, this temperature increase stresses cold-water fish species and can lead to widespread mortality.