Rainbow Glacier, on the northeast side of Mt. Baker descends from 7500
feet to 4180 feet. This glacier receives considerable avalanching from
Lava Divide, which is a ridge on the south side of the glacier.
Because of its low altitude this glacier is sensitive to spring and
fall freezing levels. Rainbow Glacier advanced between 400 and 600
feet during the 1950-1978 period of advance, and has since retreated
195 feet from the maximum moraine. Rainbow Glacier advances a short
distance each winter, emplacing a small 1-2 foot high moraine; then
retreats during the summer. With the exception of 1984, the retreat
has always been much larger than the winter advance. In 1984, a net
advance of 15 feet occurred. The last winter's moraine can be
identified by the bedrock and boulders still being coated with fine
grained glacier flour. Beyond the winter moraine most of the surfaces
have lost this cover due to the winter snowfall and melt. Mass balance
has been measured on Rainbow Glacier, from 1984 to 1990 the mean annual
balance was -0.10 m/year. Above 6000 feet snowfall by late summer
still averages 10-15 feet in thickness. Blue ice is present in small
patches as high as 6500 feet on the top of prominent knobs of the
glacier that are swept by avalanches. There is a knob at 5200 feet on
the south side of the glacier that has exposed bedrock. The slope is
to steep for ice to flow down, and melting will continue to expand the
exposed bedrock area. Another knob at 5400 feet on the north side of
the glacier is still ice covered but will probably be exposed in the
near future. The snowline cannot be identified as a single elevation;
however, below 5200 feet count on little snow.
To reach Rainbow Glacier take the Ptarmigan Ridge trail from
Artists Point to Camp Kiser. Continue across Sholes Glacier to the
Portals. From the pass betwen the two Portals descend down the east
side of the slope past the first bedrock ledge, then traverse stright
across to the Rainbow Glacier. From the 6000 foot level to the
terminus a route down the north side of the glacier to 5500 feet then
down the middle of the glacier provides nearly crevasse free travel.
The lower part of the glacier has numerous meandering streams cutting
channels as much as eight feet deep in the glacier, and numerous
moulins also exist. The slope in the ablation zone on the south side
of the glacier is moderate requiring only crampons to reach the
terminus.