STABLE ISOTOPE COMPOSITION OF STREAMWATER IN THE HATCHER PASS AREA AND KNIK RIVER, SOUTHCENTRAL ALASKA: THE INFLUENCE OF GLACIAL MELT WATER

Southcentral Alaska and many other high-latitude areas have experienced an increase in average annual temperature over the past several decades that has outpaced warming recorded in other parts of the world. This warming has led to glacial recession throughout the region and has likely caused an increase in isotopically depleted glacial melt water to streams with headwater glaciers. In order to assess the contribution of glacial melt water to the isotope composition of streamwater in a high-latitude area, weekly to bi-weekly samples of precipitation and streamwater were collected from the Hatcher Pass and Knik River areas of southcentral Alaska from Spring 2005 through early 2006. Streams sampled included the Kink River, a relatively large river sourced at the Knik Glacier; Willow Creek, a stream without a headwater glacier; and two high-elevation spring-sourced streams in the Hatcher Pass area. Additionally, several winter precipitation, glacial ice, and streamwater samples were collected throughout the region. Isotope results are more variable and relatively enriched for precipitation samples then for stream samples. Delta 18O of precipitation samples vary by approximately 9 permil, and stream samples vary by only approximately 0.5 permil. Therefore, as expected, non-glacial streams in the study area appear to be influenced more by groundwater than precipitation. Mean isotope results for small, spring-sourced streams are quite similar to mean results from precipitation samples, with a mean delta 18O of approximately of -18 permil. Knik River results suggest the isotope composition of the stream is greatly influenced by glacial melt-water, with a mean delta 18O value of approximately -19.3 permil, and did not vary by more than a few tenths of a permil over the duration of the study. Additionally, differences among winter and summer samples reflect the greater influence of groundwater in winter stream water as compared with summer. Isotopic results suggest the isotopic composition of the Knik River is more isotopically depleted water than expected if only current climate data and local isotope hydrology is considered.