WSP00036

• File Number: 8200.750
• Description: San Juan River General
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 3/1/1975
• Author: BOR
• Title: San Juan Ecology Project - Interim Progress Report - March 1975
• Water Supply Pro - Doc Type: Report/Study

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Snow Cover

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SEPT

OCT

NOV

DEC

JAN FEB

MAR APR

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Critical Snow Depth -

Figure 3. Elevation of Wintering elk as ~elated to Snow Cover, 1972-73.

evidence of impaired mobility; but it is not known
whether or not this was deleterious.

In 1970-71, all winter sightings of elk were in areas
with less than 35 em of penetrable snow cover. In
one case, elk were seen in an area with 55 em of total
snow coverj however, a hard layer (75 kg ram hardness)
25 em below the snow surface limited hoof penetration
to 15-20 em. During the second winter season
(1971-72) elk were found in areas of sparse to no
snow cover, with ODe exception. In this instance.
elk were located with a snow cover averaging 50 em.
deep. Twenty-one of 25 ram profiles taken in this
area showed a ram hardness of 1 kg or less throughout
the snowpack, which indicates light powder snow
remaining four profiles registered ram hardnesses
between 1 and 5 kg. Hoof penetration in this area
averaged between 36 em and 32 cm. This past winter
(1972-73) elk were again located in areas of sparse to
no snow cover, when this type of habitat was available.
However, during December and January Missionary Ridge
was blanketed by a complete snow cover down to and
including the valley (Fig. 3). During this period elk
were found in snow depths of 50 to 90 em. One area
of elk usage which was not reachable by investigating
teams was estimated from snow stake readings to have
as much as 110 cm of snow. In three instances, hard
layers (5 kg hardness and 6 cm thickness) were deline-
ated at the ground surface, but in general the snow-
pack was again light (1 kg). Hoof penetration in
these areas ranged from 36 em to 65 em and averaged 51
em. In areas with 70 em or more snow, there was

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These snow depth readings indicate that a depth of
35 - 40 em of penetrable snow will cause elk on
Missionary Ridge to move to more winter range with a
lesser depth. This then could be considered the
"response depthll of snow for elk in this area.
(Previous reports have labeled this the "critical
depth" of snow for elk. However, it now seems more
appropriate to save the term Ilcritical depth" for
that depth of snow which may prohibit the use of an
area by elk.) Snow depths of 40 to 60 em apparently
do not pose undue stress, and are readily traversed
for food and shelter, when more exposed areas are not
available. Areas with 70 cm or more of penetrable
snow probably represent an added winter stress, and
are usually avoided. Seventy centimeters of pene-
trable snow then represents the critical depth of
snow for elk on Missionary Ridge. '

4.6.2 Job 2. Calving (James M. Sweeney)
- Objective

Our objective is to answer the following questions:

1) Is the.spring upward movement by elk on Mission-
ary Ridge related to the receding snow line?

2) How is elk calving related to varying snowpack

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