Details - Wetlands of the Flathead Valley : change and ecological functions

Wetlands of the Flathead Valley

Title

Wetlands of the Flathead Valley : change and ecological functions

By

Newlon, Karen Rachel.

Burns, Meghan D.
Montana Natural Heritage Program.
Montana. Department of Environmental Quality
United States. Environmental Protection Agency.

Type

Book

Material

Published material

Publication info

Helena, Mont, Montana Natural Heritage Program, c2009

Notes

"January 2009"--Cover date.

Although several reports have documented loss in wetland area, few studies have addressed the corresponding change or loss of wetland functions associated with flood control, nutrient retention, and wildlife habitat. Wetlands are valued not for the area they cover but for the ecological functions they perform, so an assessment of the change in cumulative function over time is warranted. This is particularly valuable for those areas experiencing rapid land use changes that have potentially impacted wetland area, distribution, and function. The purpose of this project was to estimate wetland change in the rapidly developing Flathead Valley between 1981 and 2005 and estimate cumulative change in wetland functions. We compared historic National Wetland Inventory (NWI) wetland mapping from 1981 for the Flathead Valley with updated wetland mapping based on 2005 color-infrared aerial photography and added hydrogeomorphic (HGM) modifiers to link wetland type and wetland function. To analyze wetland change, we compared randomly selected wetlands from the original NWI with new NWI mapping created for this project. We randomly selected 10% of the one-square mile Public Land Survey System sections in each fifth-code hydrologic unit in the study area. Within the sampled area, we compared each wetland polygon in the old mapping to the corresponding wetland polygon in the new mapping, and we assigned a source of change to each polygon. In addition to changes in wetland area, we also examined changes in land cover type within the study area and within a one-kilometer buffer of each wetland polygon. To assess the functions associated with each wetland, we assigned an HGM attribute code to all wetland polygons in both the old and new wetland mapping. These HGM attributes were combined with the NWI classification attributes to yield a combination ranked on a performance scale of 1 (high), 2 (moderate), and 3 (low) for each of ten wetland functions. We used this performance ranking as a weighting factor and multiplied this weighting factor by wetland area to calculate functional units for each wetland function. We digitized nearly 132,000 acres (53,419 hectares) of wetlands within the study area. Deepwater types associated with Flathead Lake comprised over 75% of the wetland area. As expected, the majority of wetland and riparian habitats (24,255 acres; 9,816 hectares) occurred on private lands within the study area. We observed a slight overall decline of 358 wetland acres (145 hectares) between 1981 and 2005 within the study area, although estimates were highly imprecise. At the fifth-code hydrologic unit level, the greatest decline in estimated wetland area occurred in the Ashley Creek watershed with 1,366 acres (553 hectares) lost. However, most watersheds showed increases in estimated wetland area. Within the wetlands sampled, most wetland changes were attributable to natural causes such as succession. Overall anthropogenic changes in land cover type have been largely changes from Forest and Grassland/Shrub types to Urban and Agriculture types, and the Flathead River-Columbia Falls and Lake Mary Ronan watersheds have seen the largest changes with over 2,405 acres (971 hectares) converted. Land surrounding palustrine emergent wetlands showed the greatest anthropogenic change with 3,160 acres (1,279 hectares) of Open Water, Forest and Grassland/Shrub types converted to Agriculture or Urban cover types. Deepwater throughflow wetlands consisting of Flathead Lake and associated lentic wetlands comprised the largest hydrogeomorphic type in the Flathead study area, totaling 110,761 acres (44,824 hectares). Wetlands associated with lotic features covered 13,737 acres (5,560 hectares), and terrene wetlands totaled 6,800 acres (2,752 hectares). When examined by watershed, the Flathead Lake watershed contained the largest area of deepwater and lentic wetlands with 60,541 acres (24,500 hectares), and the Flathead River-Columbia Falls watershed had the largest area covered by lotic wetlands with 465 acres (188 hectares) within the study area. Comparison of wetland functional performance capacities throughout the study area showed a 38.4% loss in hydrologic functions including water storage, streamflow maintenance, and groundwater recharge. Biogeochemical functions incorporating nutrient cycling, sediment retention, and shoreline stabilization showed a slight increase, whereas functions associated with terrestrial and aquatic habitat, native plant community maintenance, and conservation of wetland biodiversity showed an overall decline of 15.4%. These changes in functional capacity were due to both natural and anthropogenic changes in wetland type and area, as well as changes in mapping conventions between the historic and updated mapping. Although our analysis showed relatively little overall change in wetland area between 1981 and 2005, we did find changes in wetland functional capacity, particularly in terms of functions related to hydrology and habitat. Additionally, analysis of changes in land cover type around wetlands showed that relatively large areas of forest and grassland/shrub have been converted to agricultural and urban types. These changes have likely affected not only the quality of these wetlands, but also the spatial extent and pattern of wetlands on the landscape, resulting in a loss of connectivity, reduced water quality, and a reduction in overall wetland integrity. We emphasize that limitations exist with any wetland mapping effort derived almost exclusively from photointerpretation techniques. Factors such as photo quality, scale, and environmental conditions at the time of photo acquisition can affect mapping accuracy. We also emphasize that the functional capacity ratings assigned to wetlands in this project are only potential capacities, and data on actual functional capacity would require extensive field checking. This analysis should be considered a preliminary assessment of changes in Flathead Valley wetlands and wetland functional capacity. Data from this analysis can provide very effective conservation tools to identify areas with the potential to perform wetland functions most effectively, allowing natural resource managers and other stakeholders to focus or prioritize their conservation and restoration efforts.