Wetland Construction

Wetlands provide many benefits:

• Provide an alternative to crops or livestock in wet or marginal areas
• Slow and store agricultural runoff
  • Reduced flooding
  • Reduced soil erosion
• Improve water quality by collecting and filtering sediment, nutrients and other pollutants in runoff
• May help recharge groundwater by slowly releasing water into the ground
• Create or enhance wildlife habitat
  • Provide food and shelter
  • Upland areas can provide habitat for important pollinator species
  • Create wildlife corridor

Site Considerations

• Naturally wet or marginal areas
  • Look for natural flow paths or low lying areas where water ponds after heavy rainfall
• Land underlain with poorly drained soils, such as clay or silty clay
  • Well drained sands and gravel are not suitable as they do not retain water
• Areas next to or within existing habitat features such as woodlots or wetlands

Conduct a test hole in the proposed wetland area to observe how water levels change over time; this will help indicate the water levels in the future pond.

Wetland Design

Catchment size measures the area that will drain into the wetland, which determines how much water the wetland will capture. Catchment size can be estimated from topographical maps or surveys, and by examining drainage maps.

In areas where the natural slope favours water ponding, an excavation scrape may be sufficient to provide definition and adequate water storage to the wetland. When the slope does not favour water containment, a berm can help hold water in the wetland. The minimum height of the berm should range between 0.3-0.6 m above the maximum proposed water level in the wetland. Standpipes or control structures can be utilized as outlets to control the water level and manage excess water.

Consideration should be given to the type (groundwater, surface or tile) and amount of water that will feed into the wetland. Large scale runoff events may threaten the integrity of the wetland and should be integrated in the overall design plan. This should include the construction of a spillway to ensure the wetland is protected against potentially damaging amounts of water.

Material excavated from the pond area may be used to construct the berm if it contains a minimum of 10% clay.

Constructed wetlands vary in shape and size, depending on the site and the desired outcome. Irregularly shaped ponds are best suited to mimic the patterns and functions of natural landscapes.

Water depth influences the wetland vegetation and wildlife species. To enhance diversity, create shallow marsh areas along the shore of the wetland, and deeper pool sections towards the middle. Variable water depths maximize habitat, but at least 70% of the pond should be less than 1 m deep.

Shallow areas:

• Waterfowl prefer water less than 2 m deep for feeding habitat
• Provide spawning beds for some fish species
• Water 0.1- 0.5 m deep around the perimeter of the wetland provides habitat for sedges and cattails

Deeper pools:

• Limit freeze-up during the winter and allow opportunities for groundwater recharge
• Fish and amphibians require water 2-3 m deep in order to overwinter
• Submergent plants can thrive in deeper pools (2-3 m)

Enhance wetland areas by maintaining a natural buffer strip around the wetland. Planting a variety of native trees, shrubs, grasses and wildflowers in the buffer will attract more diverse wildlife. Here is information on recommended species to plant.

Water levels will vary seasonally. It is common for the wetland to dry up during summer months, when precipitation is limited and the groundwater table is low.

Considerations

• Cover the upland areas of the wetland with topsoil to allow vegetation to get established
• Plant native trees, shrubs, grasses and wildflowers around the upland areas of the pond to enhance wetland function and provide habitat
  • Reduce weed pressure on young trees and shrubs by mowing or spot spraying herbicide around them
  • Take proper care to avoid spraying the leaves or stems of trees and shrubs
  • Avoid broadcast spraying herbicide around the open water.
• Plant a variety of aquatic vegetation around the wetland’s shallow shoreline to increase habitat and filter water
  • Aquatic vegetation will move in naturally over time
• Place large boulders or logs in the wetland to provide basking areas above the water surface for turtles and other wildlife

Your property may already contain a wetland that has become degraded over time. Restoring or enhancing this area can restore the wetland’s natural functions. This is an important consideration in areas within or next to wildlife habitat.

Maintenance

• Limit the growth of woody vegetation on berms, as the roots may compromise the berm’s integrity
• Floating algae mats may develop in the wetland, but can be removed and disposed of upland
• Consider removing invasive species

Contact your Conservation Authority regarding the need for permits before modifying, enlarging, redesigning, or re-establishing existing wetlands and ponds. Contact your local Conservation Authority or Duck’s Unlimited for assistance planning and designing a wetland.