Restoring Freshwater Tidal Wetland Systems
Pool spacing, channel morphology, and the restoration of tidal forested wetlands of the Columbia River, USA. Heida Diefenderfer and David Montgomery.
Large wood structures have been recognized as having an impact in both aquatic systems (rivers) and terrestrial ecosystems (old growth forests)
The nature, structure and function of large wood in tidal channels has been forgotten.
Wood in rivers can obstruct flow and sediment transport
May create pools
Anastomosing channel patterns may be forced
Forested tidal channels receive effects from both oceanic tides and terrestrial watershed processes.
High percentage of reproduction occurs in high marshes occurs on fallen trees
However, they may also float, dredging up living trees
Large wood as animal habitat
Tree depressions, when trees float off, are used by fish
Enhances macroinvertebrate richness
Tidal channels in vicinity of Gray’s River, with a 7.7 ft. tidal range, were found to have a “forced step-pool” morphology:
An observed sequence of log-jams with pools between them
And an average number of channel-widths per pool
Pool structure is reversed from that of a mountain stream
Pools are upstream of log jams
Tails are upstream of pool
This suggests that the dominant energy is associated with the flooding tide.
Implications for restoration
Density of logs in natural forest system is high
Legacy logs under diked pastures have emerged due to erosion after tidal flow has been re-connected.
More wood is needed (imported LWD)
Systems, when mature should provide wood.
Beaver positively effect functioning of such systems
Tanner, Curtis, Jeff Cordell, Jane Rubey and Lucinda Tear. 2002. Restoration of freshwater intertidal habitat functions at Spencer Island, Everett, Washington. Restoration Ecology 10(3):564-576
Spencer Island is located between Steamboat and Union Sloughs in the Snohomish estuary.
Diked in early 1900’s; used for grazing
Flooding excluded by dikes, tidegates, ditches
Southern tip is open to the Snohomish River at river kilometer 6.1
Spencer Island is 412 acres.
Restoration is limited to south 59 acres
Phalaris arundinacea (reed canarygrass) covered two thirds of this area, with some cattail
Elevation is 0.2 to 3.0 m above MLLW in the reed canarygrass areas.
Higher areas support blackberry, alder, willow, poplar and Sitka spruce.
Re-establish tidal conditions
Displace reed canarygrass
Provide habitat for juvenile salmon
Flooding not consistent with waterfowl management to north
760 m cross levee was constructed to isolate restoration area
Two 15 m and one 60 m dike breach were made in 1994 in historical south dike
This resulted in restoration of a 4 m tidal range
Some salinity was expected; none has appeared.
Pre-breach, forest covered eastern third; it subsequently began to die off.
P. arundinacea began to die off and retreat to higher elevations; deeper areas became mudflats.
Shrubs and emergents replaced forest
Permanent open water areas became drained
Juvenile fish used site: chum, coho and chinook salmon, plus other non-game fish (peamouth chub, stickleback)
Invertebrates were less diverse than in reference site, but rich in salmon prey.
Reference site 3.2 km downstream
Naturally breached between 1965-70.
Lythrum salicaria colonized parts of site, so Gallerucella beetles were introduced as a bio-control.
Hood, Greg. 2002. Application of landscape allometry to restoration of tidal channels. Restoration Ecology 10(2):213-222.
Allometry: the relationship of the parts of an organism as it grows.
In natural tidal channels, parts with predictably related measurements are:
Outlet width and depth
Channel length, perimeter and surface area
In an artificially dredged slough on the lower Chehalis River, natural slough allometry was not maintained
As a result, sediment was retained and channels filled.
Intertidal sedge habitat abundance was related to slough size for smaller sloughs, but not for larger.
Author suggests that sedge habitat in large sloughs was destroyed by log storage and transport.
Abundance of salmonid prey of terrestrial origin (aphids and adult flies) in slough surface waters was related to slough perimeter and sedge habitat.
Allometric perspective has several implications:
1. Rather than requiring similar size reference sites, allometric relationships may be used to design channels.
2. Correct initial allometry can increase ecological predictability.
Site: an excavated slough and a reference slough off of the Chehalis River across the stream from Cosmopolis.
Floodplain is a Picea sitchensis-dominated wetland with freshwater species
Marsh habitat is 0.7 to 1.5 m lower and is a monoculture of Carex lyngbyei.
Reach is tidal and oligohaline
In natural sloughs, width, depth and area of natural sloughs were positively correlated with slough length; p<0.0001.
Slough perimeter and area were correlated; p<0.0001.
Excavated slough dimensions fell outside of the 95% confidence bands of natural slough dimensions.
Excavated slough was wider, deeper and had greater surface area than natural sloughs of its length.
It is therefore unlikely to be morphologically stable.
Allometry is different in different river systems.
Willapa, South Fork Willapa, Chehalis and North River were compared.
Freshwater discharge, gradients, soils likely result in locally differing allometry.
Sedge bench size also is related to slough dimensions, except for large channels
Historical photos show heavy use of channel edges and banks for log transport and storage.
Sedge benches on larger sloughs may have been destroyed by log storage.
Aphid export was correlated with sedge bench width because they are dependent on the habitat and less mobile
Dipteran export was not correlated with sedge bench width.
Diptera rear in mudflats, sedge benches and floodplain, so are not dependent on the sedges.