Scott T. Ralston
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Problems with Sediment & Cattail in the Drift Prairie:
The prairie pothole region of North Dakota has very important wetland attributes caused by glaciations of the region. However, the glacial actions caused very different types of wetland habitat in the region. In areas such as the Prairie & Missouri Coteau, terminal moraines caused larger rolling hills and larger deeper wetlands. In the Drift Plains, glaciers created more flat terrain & ground moraines which resulted in more numerous small, shallow wetlands. These small wetlands are at a high risk due to the high influence of agriculture farming in the Drift Plains as opposed to the primary land-use of ranch/range in the Coteau. To increase crop production the small wetlands are often drained or intentionally filled in. Unintentional filling is also common when these small wetlands are farmed through or simply due to wind and water runoff erosion called siltation.
Siltation has many detrimental effects on a wetland. The filling of the wetland basins with silt reduces the quantity of water that the basin can hold thus reducing the upper basin holding capacity of the wetlands which normally can assist in preventing flooding such as in the Devils Lake watershed. The reduced water capacity also means less water for the abundance of wildlife using these wetlands.
The silt that fills these wetlands also provides a very nutrient rich medium for cattail to grow. The hybrid cattail (Typha Glauca) expresses hybrid vigor and is the most abundant emergent wetland plant species in North Dakota. In areas such as the North East Drift Plains this single species covers over 4% of the land surface. Cattail can take over a wetland and the hybrid cattail can grow in deeper water or creates floating mats which can totally choke out a wetland and create a monotypic stand. This monotypic environment has many negative effects. Cattail shades out sunlight reducing the photosynthetic ability of submergent vegetation or algae thus creating an anoxic environment. This along with little structural & plant species diversity results in decreased invertebrate diversity and predator prey interactions. Changes in the invertebrate community result in a chain reaction up the food chain and affects all animals using the wetlands.
Much work has been done using chemical control of cattail by the USDA to open wetlands and reduce roosting locations for blackbirds which depredate crops. In addition to blackbird roosts depredating nearby crops they perpetuate the cattail problem by causing nutrient loading from the defecation of thousands of birds concentrated in these wetlands. This cattail reduction has many positive impacts beyond crop protection. Studies have shown diversity and abundance of submergent vegetation, invertebrate communities, waterfowl and shorebirds increased with removal of cattail. However, chemical control of cattail does not fix the initial problem of sedimentation so if high water levels are not maintained cattail can regenerate as soon as mudflats are exposed. Repeated chemical application is needed to maintain cattail control.
Mechanical removal of sediment will remove cattail, increase water holding capacity, provide increased space & sunlight for more diverse emergent and submergent macrophytes, periphyton and metaphyton. Increase of primary species will result in a positive influence up the food chain for all wetland dependant animals, especially waterfowl & shorebirds. Cattail & sediment removal also provides a better mosaic of habitat types such as shallow open water areas for waterfowl to feed and exposed mudflats for use by shorebirds. In sediment reduced wetlands, cattail may return with other emergent species but will be more proportionate to native conditions where it will be one component of the wetland, not a monotype. In addition to the positive wildlife benefits the sediment/cattail reduction will assist in the USDA efforts to control blackbird depredation as well as decrease the spreading of more chemical in the environment.
Sediment Removal - Ovservations from a field and onsite coordinator:
As scientists we would like to think in terms of exact science and ideal situations but our field biology experience tells us that is not always the case and we must adapt as needed. However exact a soil scientist can be in determining the specific amount of true sediment in the basin is not always replicable in the heavy equipment operation for the sediment removal. We work with skilled and experienced operators but they are limited in their accuracy to within approximate inches of sediment removal. In order to sculpt a basin exactly as determined by careful soil profiles within the basin would require slow micromanaging of the equipment operators. Micromanaging the operator does not make for a happy operator and the price of the operation greatly increases due to the time it takes. Sediment removal is one of the most expensive types of wetland restorations, easily averaging $800 per acre or more when the equipment is running efficiently. If we slow down the equipment to get more exact, the price of the restoration can quickly outrun the funds allowed for the project.
The most common equipment used for sediment removal is an excavator and a bulldozer. Both of these pieces of equipment are limited in how far they can deposit the material they take out of the basin. The distance is essentially limited to within 50 feet or less from the wetland edge. If we move the material any farther the cost skyrockets because more time is taken transporting material than time used actually scraping sediment. For this reason the biologist directing the operation must evaluate the topographical landscape of each basin and decide the best location to place the material around the wetland edge. Determination of the watershed and the path water flows into the wetland is very important so the spoil piles do not impede water movement into the basin. Feathering the material out very thin may be an option if it is a small basin without much material, the material is dry and not clumpy and if the site is yet to be seeded to grass. In established grass sites the disturbed area must be kept to a minimum because these spoil piles invariably turn into invasive weed sites that must be seeded and managed. Material is normally placed on the low side of the basin or in an exploded pie type shape around the basin if there is no defined low side. Spoil piles are also usually mounded to minimize the size of the disturbed area and area for future weed management. If the sediment removal is part of a drained wetland restoration the removed sediment (and biomass) can often be used to fill in the remaining part of the drainage ditch after a solid ditch plug.
Ideally we would like to move the sediment to the hilltops where the soil was lost but this is normally not an option. Using the excavator or loader to fill dump trucks to move the material is not feasible due to greatly increased time and equipment costs. Occasionally we can use a scraper for sediment removal which can pick up and transport the material to dump in an alternative location. However, the longer the run for the scraper again the higher the time/cost. A scraper is also one of heaviest pieces of equipment in the sediment removal arsenal, therefore will not work if the soil is at all soft or wet.
Most of the experienced equipment operators have dug more soil that most biologists will ever do and although they may not know the names or functions of each layer they know general profiles and what they expect to find at certain depths. General guidelines for instructing operators to work efficiently without micromanaging them include “remove the cattail vegetation down until you don’t see the root masses/rhizomes and then go another inch or two. If you reach a color change (subsoil) you went too far and need to back off.” They will also feel a difference in the density of the material. The sediment layer is softer and scrapes out easier. The clay material within the A horizon will feel much more solid and harder to move with the equipment. Given only these instructions we have monitored several operators without telling them an exact depth to dig and they have finished the project resulting in a very similar depth removed to the depth of sediment we determined before hand with a soil profile.
Applying sound science to our restoration projects is critical, however it is also very important that we not get too theoretical and loose site of the realistic field applications and our limiting factors which are time and money. We must determine our objective of the wetland restoration. If it is only to remove “sediment” to pre-agriculture “natural” conditions the amount to remove may be very exact and can as little as a few centimeters. If the goal is to restore diversity from a monotypic cattail choked wetland, dealing in approximate inches or general depths may be more efficient and in some cases may require removing slightly more than the “true sediment” but will serve the goal the same. The problem of cattail choked wetlands is not just a result of only that sediment that has silted into the wetland during the cropping history of the last century but is compounded by hybrid vigor demonstrated by Typha glauca and the nutrient loading of fertilizers and other runoff that is not only in the true sediment but also within some of the natural top A horizon layer. Empirical observations of wetlands restored in recent years shows all wetlands return with some cattail although it is at varying densities or vigor. Wetlands where very little material was removed or soil was not removed to the depth of the rhizomes seem to have cattail return thicker than wetlands where soil was removed down to or just below the rhizomes. Some school of though (Richardson) has suggested removing down to subsoil. The science on this proposal should be pursued but something to consider on that route is the massive amounts of material and again where to put it. To remove down to subsoil would results in at least double or more the material we currently remove which we already have difficulty placing it. It would also greatly increase the cost of the project because more material is more time on the project.
What we do need is before and after science monitoring to determine if we are reaching our goals of increased diversity and if we are having any negative effects. I have spent my entire higher education and professional career working on wetlands and specifically cattail issues in this part of the county so I feel I know the subject well. My theoretical ideas and preliminary observations tend toward what we are doing is the right thing but I would love to see the science to back it up. One unanswered concern I have right now is snow catchment. I know from field experience snow caught in cattail basins can exceed 10 feet deep while snow free basins often blow free or have minimal snow. What effect does this have on the hydrology and the spring melt water? Would we be better to leave strips of cattail or rings around the basin to serve as a snow fence? Also can strips of cattail left in the basin serve as visual barriers creating opportunities for two pairs of territorial breeding waterfowl to use one pond as a pair pond?
