Freshwater mussels in California's rivers are potentially very useful as indicators of watershed health and recorders of watershed changes. We report the occurrence and habitat of mussel populations within a continuous 8-km section of the South Fork Eel River in the Northern Coast Range of California. The primary goals of our study were 1) to compile information on species composition and population density, and 2) to examine whether spatial distribution and variability were related to geomorphology and hydrology. We found numerous individuals of 2 species (Margaritifera falcata and Anodonta californiensis), with the spatial distribution of both species characterized by high variability. Mussels in this system live almost exclusively in pools (with a few in runs), near the channel banks, and especially among sedge root-mat substrate. High discharges almost certainly provide more of a constraint on the distribution and persistence of mussels in the South Fork Eel than do low summer flows, so we used the Hydrologic Engineering Center's River Analysis System (HEC-RAS) hydraulic model to estimate physical conditions during high flows when in-channel investigations were not feasible. In all flow regimes (summer, winter, 5-y flood, and the largest floods on record), mussels were found in areas of lower boundary shear stresses and lower velocities. Our study suggests that, at various spatial scales, mussels appear to be distributed in a manner that protects them from the highest flow-induced stresses.

The Greenland Ice Sheet, which extends south of the Arctic Circle, is vulnerable to melt in a warming climate. Complete melt of the ice sheet would raise global sea level by about 7 meters. Prediction of how the ice sheet will react to climate change requires inputs with a high degree of spatial resolution and improved simulation of the ice-dynamical responses to evolving surface mass balance. No Greenland Ice Sheet model has yet met these requirements.

A three-dimensional thermo-mechanical ice sheet model of Greenland was enhanced to address these challenges. First, it was modified to accept high-resolution surface mass balance forcings. Second, a parameterization for basal drainage (of the sort responsible for sustaining the Northeast Greenland Ice Stream) was incorporated into the model. The enhanced model was used to investigate the century to millennial-scale evolution of the Greenland Ice Sheet in response to persistent climate trends. During initial experiments, the mechanism of flow in the outlet glaciers was assumed to be independent of climate change, and the outlet glaciers' dominant behavior was to counteract changes in surface mass balance. Around much of the ice sheet, warming resulted in calving front retreat and reduction of total ice sheet discharge.

Observations show, however, that the character of outlet glacier flow changes with the climate. The ice sheet model was further developed to simulate observed dynamical responses of Greenland's outlet glaciers. A phenomenological description of the relation between outlet glacier discharge and surface mass balance was calibrated against recent observations. This model was used to investigate the ice sheet's response to a hypothesized 21st century warming trend. Enhanced discharge accounted for a 60% increase in Greenland mass loss, resulting in a net sea level increment of 7.3 cm by year 2100. By this time, the average surface mass balance had become negative, and widespread marginal thinning had caused 30% of historically active calving fronts to retreat. Mass losses persisted throughout the century due to flow of dynamically responsive outlets capable of sustaining high calving rates. Thinning in these areas propagated upstream into higher elevation catchments. Large drainage basins with low-lying outlets, especially those along Greenland's west coast and those fed by the Northeast Greenland Ice Stream, were most susceptible to dynamic mass loss in the 21st century

For more than a century, geomorphologists have sought a mechanistic explanation for the formation of alpine cirques, the theater-shaped basins at the heads of valleys and carved into the flanks of mountains like the Matterhorn. It is understood that glaciers in cirques scour their beds and sap their headwalls, but evidence to constrain models has been elusive. Here I present field measurements and numerical analyses of a small alpine cirque glacier that advance our understanding of the formation of cirques. My field site, West Washmawapta Glacier, sits within a cirque carved in Helmet Mountain, and is approximately 1 km long and 1 km wide, with a maximum depth of ~185 m. The glacier and surrounding cirque walls resemble a reclined armchair, with steep head and toe sections connected by a flatter central expanse. I demonstrate that the prevailing view of cirque glaciers - in which the ice mass rotates rigidly above an arcuate bed - is not applicable. Instead, the glacier behaves much like larger temperate glacier systems, with basal stresses that tend toward 105 Pa everywhere. Partitioning between internal deformation and basal sliding is a function of the spatial variation of ice flux and basin geometry. Basal sliding rates are minimum beneath the glacier center, in the deepest part of the cirque bowl. Along the northern margin and above the stoss side of the riegel, however, basal sliding accounts for more than 50% of the surface velocity. The `classic' cirque form found at Helmet Mountain is maintained by erosion of the glacier bed and transport of loose debris away from the headwall by the glacier. Using a sediment budget approach, we show that over the past few centuries, the cirque has elongated and deepened at roughly equivalent rates of order 1 mm/yr. In 2007, we measured a proglacial stream sediment flux between 70 and 1840 tons per year at the basin outlet, a third of which left the cirque in a period of two days early in the meltseason. Using a combination of remotely-sensed and on-site measurements, I estimate that subaerial headward retreat of the headwall occurred at ~1.3 mm/yr (0.2 - 5 mm/yr). I propose that the steep bedrock slopes characteristic of the headwall, which encourage rockfall and snow avalanches, result from undermining by rock fracture and glacial plucking in the bergschrund. A suite of environmental measurements taken in the bergschrund for nearly two years demonstrate subfreezing temperatures, rather than diurnal fluctuations above and below zero, are the norm. I use my temperature measurements, coupled with a numerical description of rock fracture by ice segregation, to show that the bergschrund is a favorable environment for shattering of rock. Only within the bergschrund can periglacial weathering and glacial entrainment conspire to undermine the headwall and thereby play a pivotal role in cirque development.

Taylor Glacier, an outlet of the East Antarctic Ice Sheet, flows through the Transantarctic Mountains and terminates in the Dry Valleys. Understanding how this glacier fluctuates is important for studies of glacial geology, paleoclimate, ice dynamics, and ecology. Sublimation is the primary mass-loss process for most of the glacier. Four years of specific balance measurements from the ablation zone show sublimation rates up to 40 cm per year. We used data from an array of weather stations as inputs to a model for latent heat flux and hence sublimation rate. Calculated and measured ablation rates agree to within uncertainties, indicating that wind speed and vapor pressure gradient (a function of temperature and humidity) are the governing variables, as expected from theory. Measurements and model results together allowed us to examine the spatial and temporal variations of sublimation on the glacier. On average, sublimation is about two times faster in summer than winter. Rapid sublimation occurs during storms and katabatic wind events, but such periods contribute less to the annual total than do slow, persistent losses. Spatially, sublimation reaches a maximum midway along the glacier, where descending surface air currents are focused by the topography of the aptly-named tributary, Windy Gully.