HYDROLOGIC PROCESSES SUPPORTING FENS, MEADOWS AND RIPARIAN ZONES IN THE CORDILLERA OF WESTERN NORTH AMERICA

Wetlands, including fens, meadows and riparian zones are characteristic ecosystems of mountain regions throughout western North America. Relatively little is known about the hydrologic regimes that create, support and differentiate these ecosystem types, and how climate influences their hydrologic regime across the region. In addition, little is known about the role of ground water vs. surface water in supporting these ecosystems. In this talk I review the hydrologic and geomorphic processes that support and distinguish these ecosystem types. Fens form where perennial soil saturation allows peat accumulation processes on the time scale of millennia. Contrasts in the abundance of summer precipitation are striking across the region and control the abundance of fens. The Sierra Nevada, where fens are relatively rare has largely winter precipitation, while fens are more common in the southern Rocky Mountains where both strong summer monsoonal rains and large snow packs occur. Meadows form in sites with seasonally saturated soils, and support soils and vegetation distinctive from fens, and they generally are much more common on the landscape. Both fens and meadows are almost entirely ground water driven, as will be demonstrated from our long-term studies in many parts of the western US. Even riparian areas in mountain regions can be ground water fed, with streams being strongly gaining. These three ecosystem types nicely illustrate the hydrologic variation in mountain ground and surface water flow systems and their contribution to regional biodiversity.