Water Quality Management: Wetlands
This page summarizes potential changes in key water quality attributes that could affect inland and coastal wetlands. Links to related information are also provided (links open in a new tab).
Background
Climate change effects on surface and groundwater hydrology, sea level rise and pollutant loading present a risk to wetland protection, restoration and management, including compensatory mitigation efforts.
- Overview of Wetlands Programs
Wetlands are among the most productive ecosystems in the world and provide a range of beneficial water quality and ecological services. Inland wetlands filter water, recharge water supplies, reduce flood risks, and provide fish and wildlife habitat. Coastal wetlands, including both saltwater and freshwater wetlands, can store floodwaters, buffer storm surge, and assist in erosion control. They provide habitat for many species, including some that are federally threatened and endangered.
EPA programs support the protection and management of wetlands in four general areas: monitoring and assessment, regulation, voluntary protection/restoration and establishing water quality standards for wetlands (see box with "Related Links"). EPA’s National Wetland Condition Assessment (see box with "Related Links") addresses the current status and trends in wetland occurrence and ecological condition, including identification of stressors most commonly associated with poor condition. Through the Enhancing State and Tribal Programs initiative (see box with "Related Links"), EPA provides technical and financial support to States and Tribes to support the development of effective programs for wetland protection and management. Section 404 of the Clean Water Act (see box with "Related Links")establishes a program to regulate the discharge of dredged or fill material into waters of the United States, including wetlands. The basic premise of the program is that no discharge of dredged or fill material may be permitted if a practicable alternative exists, and that compensatory mitigation (e.g., the restoration, creation, enhancement, or in certain circumstances preservation) be provided for all unavoidable impacts.
Key Attributes
Figure WPr-1 shows the system level relationships among major climatic and other drivers, watershed processes, and hydrologic and water quality attributes relevant to EPA water programs.
While any unanticipated change in hydrology or water quality could affect inland and coastal wetlands, the following attributes are key concerns:
- Water Availability/Hydroperiod: Surface and groundwater hydrology determine the structure and function of wetland ecosystems. Any changes in hydrology affecting the availability of water could affect the timing and flow of water into and within wetlands, including the seasonal dynamics and spatial extent of saturated soil and open water conditions.
- Sea Level Rise/Coastal Flooding: Increases in sea level (or relative sea level) and storm surge can erode shoreline, cause coastal flooding, and poses risks to the spatial distribution and ecological condition of coastal wetlands, and in some cases could turn coastal wetlands into open water.
- Salinity/Saltwater Intrusion: Sea level rise and increasing storm surge can cause salt water intrusion to coastal aquifers, waterbodies, and reservoirs, presenting a risk to coastal wetlands.
- Nutrient Loads: Increased nutrient loading to inland and coastal wetlands could alter wetland trophic status and productivity, presenting a risk to aquatic communities and ecosystem health.
- Sediment Loads: Altered sediment loading to inland and coastal wetlands could alter accretion/erosion rates and, therefore, wetland morphology and habitat, threatening aquatic communities and ecosystem health.
Anticipated Changes in Key Attributes
Aniticpated climate change effects on key hydrologic and water quality attributes relevant to inland and coastal wetlands are summarized below. Regional differences are described with reference to the 6 contiguous U.S. regional divisions used in the 2014 Third U.S. National Climate Assessment. For additional information, including literature cited, readers should refer to the detailed reviews under “Water Quality Topics”.
- General Watershed and System Changes
Changes in Land Use, Water Management Infrastructure and Other Drivers
Most watersheds in the U.S. have been altered by human activities including urban and agricultural land use, dams, diversions, withdrawals and other activities. Land cover conversion to urban and agriculture has a significant impact on watershed hydrologic processes together with pollutant sources and transport into and within waterbodies. The effects of these changes on pollutant loading vary in different watershed settings and can be mitigated through management interventions or “best management practices”, the exact nature and forms of which depend on site specific conditions and characteristics including flow, slope, soil type(s), vegetative cover, and other features. Dams, reservoirs and other structures and practices have been deployed to provide storage of freshwater, generate hydropower, and assist in flood prevention, simultaneously modifying natural hydrodynamic characteristics. Extraction of water (surface and ground) to meet growing human demand also affects streamflow and water quality. Climate change will interact with these and other factors in different regional and watershed settings to affect streamflow and water quality.
Changes in Hydrology
Watershed hydrology can be characterized as a water balance; with inputs from precipitation balanced by outputs as evapotranspiration and runoff. Climate change affects the water balance by altering the amount and seasonal timing of precipitation delivered to a watershed, and by affecting losses through evapotranspiration, which is a function of air temperature (as well as humidity, solar radiation and wind). Climate change also affects hydrologic processes that govern groundwater recharge and runoff (e.g., infiltration). Projected climate change in most U.S. regions is anticipated to include warming temperatures, with less certain and regionally variable changes in the amount and seasonal timing of precipitation (see the following summary of U.S. Climate Change). Generally, historically wet regions (e.g., the northern and eastern U.S.) are expected to become wetter with corresponding increases in total runoff/streamflow; whereas historically drier regions (e.g., primarily the arid southwest, southern Great Plains, and parts of the southeast) are likely to become drier. In the Northwest region small changes in total streamflow relative to the range of historical variability are anticipated, with no consistent direction of change. Variability in hydrologic response within any region is expected as a result of differences in local factors that influence hydrologic response including: local geology, topography, soil type, and vegetation.
A warming climate is expected to increase the risk of heavy precipitation because warmer air has the potential to hold greater amounts of water vapor. Increases in heavy precipitation events (magnitude and frequency) and the potential for longer dry periods between precipitation events could lead to an increased frequency of extreme high and low flow events. Increases in winter temperature in the northern U.S. and mountain west are expected to cause more winter precipitation as rain, reduced winter snowpack, and snow melt earlier in the year. These changes, if realized, are likely to drive increased winter-spring streamflow and decreased summer-fall streamflow (see “Streamflow”). Warmer air temperatures are also likely to drive increased evapotranspiration and potential for drought periods, particularly in summer. Such changes could have direct and cascading effects on streamflow and water quality.
Changes in Pollutant Loading (Nonpoint Source and Point Source)
Northern and eastern regions of the U.S. that experience increases in precipitation and runoff, including heavy precipitation events, could see increases in nonpoint source (NPS) loadings of nutrients, sediment and potentially pathogens from upland sources to waterbodies (e.g., see “Nutrients”). In the Southwest and southern Great Plains, despite the likelihood of drier conditions, increases in the frequency of heavy precipitation could lead to episodic increases in NPS loadings of nutrients, sediment and pathogens. The effects of climate change on pollutant loading will vary in different watershed settings, and will interact with local changes in urban and agricultural land uses, pollutant sources, and other human activities. Changes in effluent discharges from point sources could have a significant effect on pollutant loading and concentrations in waterbodies.
- Changes in Water Availability/Hydroperiod
Climate change could alter the occurrence, distribution and movement of water flow into and within wetlands, including the seasonality of wetting/drying, inundation period and spatial extent of saturated soil and open water conditions. Climate change will affect inland wetlands through the combined changes in precipitation and temperature, leading to changes in runoff and other hydrologic processes that govern the spatial and temporal distribution of surface and groundwater flow. Groundwater levels are a major driver of wetland hydrology and influence the frequency and duration of wet conditions, which determine the structure and function of wetland ecosystems. In wetter regions (e.g., northern and eastern regions), increases in winter-spring precipitation could increase the length and depth of flooding of inland wetlands, especially in depression wetlands between arid and mesic climates (e.g., Prairie potholes in the northern Plains and Midwest). These changes could offset any depletion from summer dry periods and increased evapotranspiration; however, such seasonal shifts may have substantial impacts on wetland condition and function. In drier regions (Southwest and southern Plains), decreases in precipitation and increasing frequency or duration of drought could decrease groundwater recharge and lead to less-frequent flooding of existing wetlands. This trend has already been observed in parts of the Southwest, and is suggested to intensify as the century progresses. Increases in groundwater extraction would exacerbate decreases in groundwater, particularly in areas like the southern Great Plains and Southwest U.S. where water availability is limited. If realized, these changes present a risk to wetland aquatic life and ecosystem health. Changes also would affect wetland protection programs such as siting and design decisions for created and restored wetlands in compensatory mitigation. Climate change effects on water availability will interact with land use, water management infrastructure and other human activities.
- Changes in Sea Level Rise/Coastal Flooding
Sea level rise and storm surge can lead to coastal flooding and present a risk to the spatial distribution and ecological condition of coastal wetlands. The effects of sea level rise on coastal systems are driven by relative sea level rise. Relative sea level rise is influenced by changes in the ocean water volume and changes in the land surface elevation, either natural or in association with human activities (e.g., oil or groundwater extraction). The East and Gulf coasts are expected to experience relatively high rates of SLR compared to the west coast (particularly the Pacific Northwest) (see “Salinity”). Interactions between SLR, increases in heavy precipitation, coastal storm severity and tidal dynamics are likely to increase the risk of coastal flooding and storm surge events. As sea level increases, periodic inundation and increasing salinity could affect the spatial distribution and ecological condition of coastal wetlands. Coastal wetlands provide important services including flood storage, buffering of storm surge, and erosion control. Increased storm surge combined with relative SLR can lead to marsh erosion and loss. Increased organic matter mineralization and decreased productivity are likely to alter the balance between marsh accretion and the combination of subsidence and SLR, leading to net loss of wetland area as well as decreases in wetland water quality.
- Changes in Salinity/Saltwater Intrusion
Sea level rise and increasing storm surge can cause saltwater intrusion to coastal aquifers and waterbodies, presenting a risk to coastal wetlands. Changes to salinity and saltwater intrusion in coastal areas are driven by sea level rise (SLR), storm surges, and the volume and timing of freshwater runoff. The position of the freshwater/saltwater interface in coastal rivers and estuaries is an important zone of biogeochemical transformation and critical habitat for coastal wetlands. Increases in salinity can decrease the pycnocline depth and increase the volume of water isolated from reaeration and therefore increase the risk and extent of hypoxia, but this will vary by estuary depending on local hydrodynamics. Research suggests that the position of the saltwater-freshwater interface in some coastal rivers is likely to move upstream due to SLR over the next century, potentially impacting water quality and wetlands (see “Salinity”). The effects of saltwater intrusion are projected to be greater on the East and Gulf coasts, where SLR is projected to have a higher impact than the West coast, especially Pacific Northwest, and Gulf coasts. Potential increases in salinity in coastal rivers are more likely during summer-fall in many locations due to projected decreases in streamflow (freshwater inflows) at this time of year. Saltwater intrusion and increases in salinity, if realized, could affect the spatial distribution and ecological condition of coastal habitats and wetlands.
- Changes in Nutrient Loads
Excess nutrients in inland and coastal wetlands can alter wetland trophic status and productivity, presenting a risk to aquatic communities and ecosystem health. Changes in nutrient loads to wetlands can be driven by precipitation and runoff (which transport nutrients from upland sources to waterbodies and from instream sources through channel erosion) and watershed biogeochemical cycling; moreover, loss of wetlands would remove their nutrient uptake, storage, and removal services, exacerbating downstream loading. In the northern and eastern U.S., projected increases in total precipitation and runoff, if realized, would increase the risk of excess nutrient loading (TN and TP) from upland sources to waterbodies (see “Nutrients”). In the Southwest and southern Plains, generally drier conditions and reduced runoff, particularly in summer, could lead to decreases in nutrient loads. In all regions, increases in heavy precipitation and runoff events could increase the risk of episodic high nutrient loading to waterbodies. Increased nutrient loading, particularly in summer, can drive increases in primary productivity, increased risk of algal blooms and associated decreases in dissolved oxygen. Climate change effects on nutrient loading to wetlands will interact with land use and other human activities that affect nutrient sources, hydrologic processes and transport. Excessive inputs of nutrients to wetlands could lead to eutrophication affecting aquatic life and other ecosystem services.
- Changes in Sediment Loads
Changes in sediment loading to inland and coastal wetlands can alter wetland morphology, hydrology and physical habitat, presenting a risk to aquatic communities and ecosystem health. Sediment loading to wetlands is mainly driven by precipitation and runoff, together with geology and soil type, land disturbance and vegetative cover, and other factors affecting upland soil erosion and transport to water bodies. Instream bed/bank erosion also contributes sediment to wetlands that receive direct sediment inputs from rivers and streams (e.g., riverine, estuarine). Most studies suggest changes in stream sediment loads that correlate with future changes in precipitation and streamflow (see “Sediment”). In northern and eastern regions of the U.S., potential increases in precipitation and runoff, including increases in winter rainfall, would increase sediment loading from upland sources to waterbodies. Changes in the seasonal pattern of sediment delivery (e.g., increases in winter/spring and decreases in summer) may also occur. For some coastal marshes, increased sediment loading would supplement marsh platform accretion, helping to balance relative SLR. In the Southwest and southern Plains, drier future conditions could lead on average to decreases in sediment loads, although more severe episodic storms could be associated with high sediment loading events. Increases in heavy precipitation events in all regions would drive episodic increases in sediment loading, as well as increased instream bed/bank erosion. Climate change effects on stream sediment loading to wetlands will interact with upstream land use, vegetation cover, water management infrastructure and other human activities that affect sediment sources, hydrologic processes and transport. Changes in sediment loading, if realized, would also increase the transport into wetlands of pollutants that adsorb to particles.
Summary Table
Table Wet-1 is a summary of potential regional changes in priority hydrologic and water quality attributes that could affect inland and coastal wetlands. For each attribute, a direction of change is indicated, together with an estimated level of understanding. The "Level of Understanding" assigned is qualitative and based on the authors’ subjective interpretation of 2 factors: the strength of evidence, and the agreement of evidence among literature identified in this review (see "Level of Understanding"; note: all links will open in a new tab or window). Readers should refer to the technical literature reviews in “Water Quality Topics” for additional details and a listing of individual studies considered in this review.
Tools and Case Studies
Water managers across the United States are anticipating, planning, and preparing for the impacts of climate change. A number of tools and case studies relevant to TMDLs and Watershed Protection are listed below. See "Tools and Data" and "Case Studies" for additional tools and case studies.
- Adaptation Case Studies
- USEPA: Climate Change Adaptation Resource Center (ARC-X)
- Maryland Analyzes Coastal Wetlands Susceptibility to Climate ChangeEXIT
- Southwest Florida Assesses Salt Marsh Vulnerability to Sea Level RiseEXIT
- Pew Center on Global Climate Change: Regional Impacts of Climate Change: Four Case Studies in the United States (80 pp, 96 K, About PDF)EXIT
- USGS: Modeling Sea-Level Rise in San Francisco Bay EstuaryEXIT
- Maryland Department of Natural Resources (MDDNR): Maryland Coastal Resiliency AssessmentEXIT
- Chesapeake Bay Program: Management Strategies and Work Plans Dashboard: see wetlandsEXIT
- The Partnership for the Delaware Estuary (PDE): Marsh Futures toolEXIT
- National Estuarine Research Reserve System: Climate Change Vulnerability Assessment Tool for Coastal Habitats (CCVATCH)EXIT
Note: This page contains detailed technical information . Please refer to "Contact Us" if additional assistance is needed.