GROUNDWATER INVERTEBRATES INDICATE THE LARGE SPATIAL EXTENT OF THE HYPORHEIC ZONE IN MONTANE FLOODPLAIN AQUIFERS

A diverse community of invertebrates occurs in some montane floodplain aquifers, where highly transmissive coarse-grained sediments result in hyporheic exchange extending well away from the active river channel. In the Flathead River, Montana, over eighty species of hyporheic invertebrate fauna are known, many of them previously undescribed species prior to 1988. Large-bodied hyporheic stoneflies are abundant and widely distributed as larvae. However their amphibitic life history dictates that they must emerge as adults and lay eggs near the main river channel. At our study site along the Middle Fork, Flathead River, amphibitic stoneflies emerge in large numbers from nearly all of the sampling wells throughout the flood plain, often hundreds of meters from surface waters. The patterns of distribution and emergence of these stoneflies indicate the large spatial extent of the hyporheic zone. Furthermore, they imply the existence of a network of preferential flow zones, where carbon, nutrients, and oxygen are supplied from the main channel, and through which these invertebrates can disperse. We report on the distribution patterns and community composition of groundwater invertebrates from our extensive floodplain scale well network. We compare these patterns within the framework of surficial cues such as hyporheic position (from river infiltration, to exfiltration into springbrooks or ponds), and floodplain cover type (active gravel bars to the mature, forested riparian zone). We have also developed a groundwater mesocosm that will help refine our estimates of organismal density and secondary production, identify sources of carbon that feed these systems, and conduct experimental manipulation of groundwater communities.