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

Wetlands of the Gallatin Valley

Title

Wetlands of the Gallatin 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

"December 2009"-cover.

Digital wetland mapping provides important information on the type, location, and extent of wetlands within a given region. Comparing historic mapping with updated mapping provides a unique opportunity to examine potential changes in wetland density and distribution due to both natural and anthropogenic causes. In addition to documenting changes in wetland area, comparing spatial datasets allows us to track change or loss of wetland functions such as flood control, nutrient retention, and wildlife habitat. This report focuses on the Gallatin Valley and surrounding area, typical of many rapidly growing regions in the West with increasing land conversion, subdivision, and residential development. Our objectives were to quantify changes in wetland ex-tent and function in our study area and to estimate cumulative change in wetland functions. The project required us to produce new digital wetland maps at a 1:12,000 scale, using 2005 aerial imagery at 1-meter resolution. This was done as part of National Wetland Inventory (NWI) updating, following current federal standards. To analyze wetland change, we compared randomly selected wetlands from the original NWI, completed in 1984 and 1988, with the new NWI mapping created for this project. We randomly selected 25% of the one-square mile Public Land Survey System sections in each subwatershed in the study area using a spatially balanced random sampling approach. Within the sampled area, we compared each wetland polygon in the old map-ping to the corresponding wetland polygon in the new mapping, and we assigned a source of change to each mapped wetland. To assess the functions associated with each wetland, we analyzed the landscape position, landform, waterbody, and water flow paths for each wetland. We assigned hydrogeomorphic (HGM) modifiers 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 that could then be ranked on a performance scale of 1 (low), 2 (moderate), and 3 (high) for each of ten wetland functions (water storage, streamflow maintenance, groundwater recharge, nutrient cycling, sediment retention, shoreline stabilization, native plant community maintenance, terrestrial habitat, aquatic habitat, and conservation of wetland bio-diversity). 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 also completed a wetland landscape profile for each sixth code hydrologic unit that provides a broad landscape characterization of wetlands across the project area. We digitized 56,822 acres (22,995 hectares) of wet-lands and 28,210 acres (11,416 hectares) of riparian habitat within the change detection analysis area. Palustrine emergent wetlands covered the greatest area with over 28,380 acres (11,485 hectares). The majority of wetland and riparian habitats (57,358 acres; 23,212 hectares) occur on private lands within the analysis area. Overall, we observed an increase of 4,221 mapped wetland acres (1,708 hectares) between 1980's and 2005 within the study area. Wetlands associated with lotic features comprised the largest hydrogeomorphic type in the Gallatin project area, totaling 39,454 acres (15,967 hectares). Wetlands associated with deepwater and associated lentic features covered 824 acres (333 hectares), and terrene wetlands totaled 3,256 acres (1,318 hectares). Comparison of wetland functional performance capacities throughout the Gallatin project area showed an overall 73.5% gain in hydrologic functions that include water storage, streamflow maintenance, and groundwater recharge. However, we mapped over five times more acres of lotic wetlands using higher resolution 2005 imagery, which contributed to this apparent gain in hydrologic function. Biogeochemical functions incorporating nutrient cycling, sediment retention, and shoreline stabilization showed an overall increase of 24%. Functions associated with terrestrial and native plant communities showed a combined decline of 7.3%, whereas aquatic habitat and conservation of wetland biodiversity showed a combined increase of 9.7%. Our analysis shows an overall increase in wetland area between mapping completed in the 1980's and new mapping from 2005 aerial imagery. Many wetlands mapped as palustrine scrub shrub in the 1980's are now palustrine emergent wetlands. Examination of the aerial imagery revealed that much of this change is attributable to agricultural changes (e.g., livestock grazing, stream dewatering, and conversion to hay pasture). Additionally, some scrub shrub historically mapped as wetland was mapped as riparian scrub shrub in the new mapping. Although our estimates indicate that few actual wetland acres were lost between the 1980's and 2005 mapping efforts across the entire project area, concentrated wetland losses occurred in a few areas. In particular, much of the wetland change in the areas immediately around Bozeman was attributable to urban and rural development. This area has seen rapid growth and much of the valley bottom locations along the East Gallatin River have been subdivided. The wetland landscape profiling also revealed that the areas around Bozeman contain wetlands with the potential for high performance of several wetland functions, including groundwater recharge, streamflow maintenance, water storage, sediment retention, and terrestrial habitat. Continued impacts to wetlands in these important areas will reduce the ability of wetlands to perform these functions, potentially resulting in ecological and economic losses in these areas. It is also important to note that differences between the scales of the imagery used in the historic and new mapping products make accurate quantification of wetland change problematic. The historic wetland mapping was digitized at a 1:58,000 scale and exhibits considerably more spatial error than the current mapping that was digitized at a 1:12,000 scale. Factors such as photo quality, scale, and environ-mental conditions at the time of photo acquisition can also affect mapping accuracy. Digital wetland maps are static and may not reflect the dynamic nature of wetlands subject to drastic annual and seasonal fluctuations in size and distribution. We also emphasize that the functional capacity ratings assigned to wetlands in this project are only potential capacities. Data on actual functional capacity would require extensive field work and assessment. This analysis should be considered a preliminary assessment of changes in the Gallatin Valley wet-lands and wetland functional capacity. Data from this analysis can provide very effective conservation tools to identify areas with the potential to per-form wetland functions most effectively, allowing natural resource managers and other stakeholders to focus or prioritize conservation and restoration efforts.