Hydrologic Diversity in Glacier Bay Alaska: Spatial Patterns and Temporal Change

Abstract

A high spatial resolution (250 m), distributed snow evolution and ablation model, SnowModel, is used to estimate current and future freshwater runoff into Glacier Bay, Alaska; a fjord estuary that makes up part of Glacier Bay National Park and Preserve (GBNPP). The watershed of Glacier Bay contains significant glacier cover (tidewater and land-terminating) and strong spatial gradients in topography, land cover, and precipitation. The physical complexity and variability of the region produces a 15 wide variety of hydrological regimes, including rainfall, snowmelt, and ice-melt dominated responses. The historic mean annual runoff into Glacier Bay proper is found to be 24.5 km 3 yr -1 , with a peak in July, due to the overall dominance of snowmelt processes that are largely supplemented by ice-melt. Future scenarios (2070-2099) of climate and glacier cover are used to estimate changes in the hydrologic response of Glacier Bay. Under the RCP 8.5 scenario, the mean of five climate models produces a mean annual runoff of 27.5 km 3 yr -1 , a 12.2% increase from historical conditions. When spatially aggregated over the 20 entire bay region, the future seasonal hydrograph is flatter with weaker summer flows and higher winter flows. The peak flows shift to late-summer and early-fall and rain runoff becomes the dominant overall process. The timing and magnitudes of modeled historic runoff are supported by a freshwater content analysis from a 24-year, CTD-based oceanographic dataset from the U.S. National Park Service's Southeast Alaska Inventory and Monitoring Network (SEAN). Individual watersheds display a variety of changes, depending upon total glacier coverage, elevation distribution, landscape characteristics, and seasonal changes to the 25 freezing line altitude.

A Classification of Streamflow Patterns Across the Coastal Gulf of Alaska

“…Future increases to the freezing line altitude will play a key role in reorganizing the streamflow patterns of coastal GOA within and across watersheds. In Glacier Bay—a steep and heavily glacierized fjord within the GOA drainage area and approximately 80 km west of Juneau, Alaska (Figure )—runoff forecasts predict that the annual proportion of snowfall contributing to total freshwater runoff across all watersheds will decrease from 58% to 24% by the year 2100 (Crumley et al, ). This dynamic is accurately reflected in our fuzzy analysis, where the majority of transitional watersheds in Glacier Bay share snow/glacier metaclasses (Figure ).…”

“…The elevation profiles of individual watersheds are also critical context with which to assess streamflow change at smaller spatial scales. For example, the Dundas River, flowing through a relatively low-elevation watershed in Glacier Bay (classified as Snow-IV; Table 5), is likely to see decreased snowfall amounts at the end of this century, as warming air temperatures are predicted to increase winter freezing line altitude approximately 300 m (Crumley et al, 2019). In contrast, winter freezing line altitude increases may be less important to the overall shape of the hydrograph in steeper and larger glacier-driven watersheds (Glacier-I and Glacier-II; Table 5), where most of the drainage area will remain within winter freezing air temperatures.…”

“…In contrast, winter freezing line altitude increases may be less important to the overall shape of the hydrograph in steeper and larger glacier-driven watersheds (Glacier-I and Glacier-II; Table 5), where most of the drainage area will remain within winter freezing air temperatures. As the twenty-first century progresses, the summer hydrograph of these two glacier-driven classes may steadily flatten in summer due to decreased glacial volume while the fall and winter hydrograph may increase due to greater amounts of precipitation falling as rain (Beamer et al, 2017;Crumley et al, 2019). Climate models generally agree that winter precipitation will increase throughout coastal GOA, but uncertainty exists among models regarding whether summer precipitation will increase in the region (Maloney et al, 2014).…”

Modeling coho salmon (Oncorhynchus kisutch) population response to streamflow and water temperature extremes

“…The winter flood dynamics of watersheds such as those found in southern coastal Alaska are likely to become increasingly complicated in the future. Over the remainder of the 21 st century, scientists expect snowline elevations to continue rising, which will shift the hydrology of many watersheds from primarily snow-fed to primarily rain-fed inputs (Beamer et al 2017;Littell et al 2018;Crumley et al 2019). A rising snowline elevation also increases the potential for greater amounts of precipitation falling as rain during the winter.…”