Geoscience Reference
In-Depth Information
5.5 Discussion
5.5.1 Photogrammetric and GIS Analysis of Rates of Flow
Velocities for the California rock glacier fall within the typical range of most pub-
lished flow rates (1-100 cm/yr) (Barsch
1996
). On Galena Creek rock glacier in
the Absaroka Mountains of Wyoming, Potter et al. (
1998
) measured rates of flow
between 14-80 cm/yr from the 1960s to 1990s. In the Elk Mountains of Colorado,
flow rates averaged 63 cm/yr from 1964-1968 (Bryant
1971
). Front Range rock
glaciers have rates of flow that typically average 10-20 cm/yr from the late 1970s
to late 1990s, much slower compared to an average of 57 cm/yr for California rock
glacier from 1983 to 1998 (Janke
2005b
). The maximum velocities on California
rock glacier, however, are insignificant in comparison with some > 300 cm/yr dis-
placements that have been observed on the Fireweed rock glacier in Alaska and on
some rock glaciers in the Austrian Alps (Bucki and Echelmeyer
2004
; Krainer and
Mostler
2006
). If large volumes of rock and ice are moving down slope at greater
rates, this could present hazards for nearby alpine communities.
Near the head of California rock glacier, a section of decreasing flow rates
showed a marked difference compared to an interior region (Fig.
5.5
). Interestingly,
the change in velocity seems to reflect two different sources of talus and snow input.
The talus source on the southern edge of the headwall is associated with increasing
rates of flow, whereas a talus source located on the northern edge of the headwall is
experiencing decreasing rates of flow. Thinning is apparent in each of these regions,
which may be the result of extensional flow or a decaying internal ice structure
(Fig.
5.6
). Rates of flow are typically greater near the head of the rock glacier
(Benedict et al.
1986
; Janke
2005b
). This is also the case for the California rock
glacier. More loading of rock adds weight to the head. When combined with greater
slopes near the head, greater flow rates are produced.
The rock glacier midsection showed a decrease in horizontal velocities over the
two periods (Fig.
5.5
). This may be the result of ice thinning, which inhibits the rock
glacier from flowing as quickly. No significant change in elevation was observed in
this region, however, except near the northern edges of the rock glacier (Fig.
5.6
).
Perhaps ice from the head of the rock glacier, which has shown a slight increase in
horizontal velocity, has propagated down glacier and has slowed from compression.
5.5.2 GPS Analysis of Flow Rates
The GPS point measurements were also compared with photogrammetric estimates
of flow by overlaying points on the interpolated grids for 1983-1988 and 1988-
1998 (Fig.
5.7
). Points 1-3 showed a steady decline in surface velocity from 1983 to
2008. Interestingly, points 5-7, which are located near the toe, exhibited an increase
in surface velocity from 2003 to 2008. Unlike temperate glaciers, the temperature
of ice in a rock glacier has been shown to affect rates of flow more so compared
