Using Stable Isotopes of Water to Determine Contribution of Glacial Melt to Stream Flow and Plant Water Use in Grand Teton National Park

Abstract

The loss of mountain glaciers will have a substantial impact on streamflow, and will consequently impact human and ecological systems. Grand Teton National Park is renowned for its glaciers, namely the Grand and Middle Teton glaciers, and their extent is rapidly declining. Not well understood is the contribution of glacier meltwater to streamflow and the importance of the rivers for riparian plant water use. The goals of this study were to determine the relative contribution of snowmelt and glacier meltwater to streamflow in the Middle and Grand Teton watershed, and the contribution of these watersheds to Cottonwood Creek, a tributary of the Snake River. Additionally, the contribution of river water vs unsaturated alluvial soil water to riparian plant water use was quantified. A field campaign was conducted in mid-August, 2008, where snowmelt, river, plant stem, and soil samples were collected from the Middle Teton (MT) and Grand Teton (GT) watersheds, and Cottonwood Creek. Water was extracted from the plant stems and soils. All water was analyzed for stable isotopic content (8D and 8180). The data were analyzed with a mixing model in a Bayesian framework, which allowed for full accounting of uncertainty in the data and model. Despite a fairly large and persistent snowpack, glacier meltwater contributed nearly 92% and 79% to stream flow in the Grand and Middle Teton watersheds during this period of time. However, the range of glacier meltwater contributions was quite large because of the similar isotope values of snowmelt and glacier melt water. The MT watershed contributed 29% of the water to Cottonwood Creek and the GT watershed contributed about 16%. The remammg contribution was likely outflow from Jenny Lake. Nearly 28-55% of plant water was derived from the river, so the bulk of their water was derived from unsaturated alluvial soils. This is likely because plant roots require well-drained soils, so they do not have their roots directly in the river. These findings suggest that further decline in the Grand and Middle Teton glaciers will reduce streamflow within each watershed and streamflow in Cottonwood Creek. The latter effect may impact the Snake River, which is a major tributary of the Columbia River. Further, loss of the Grand and Middle Teton glaciers will impact riparian plant water availability, and any impact on growth may affect other species, such as birds, which are important animals for Park visitors.