WMOD00002

• Applicant: Western Weather Consultants
• Project Name: San Juan
• Date: 11/1/1984
• Weather Modification - Doc Type: Application

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TO SNQl.i COVER
Ste i nhoff

ABSTRACT

A 4-year study was conducted in the San Juan Houn-
taima! southwestern Colorado to determlne t~elTect5
of snow on the distribution and calving~k (Cervus-
canadensis), Seventy-five aerial surveys tes~lted in
1262 winter and 2325 spring elk sightings. Measure-
ments of ~he snowpack were obtained in areas of winter
elk sightings and at 31 permanent snow stake sites to
quantify the c~itical depth of snow fOT elk and to
estimate winter range. Ground surveys provided supple-
mental information to the spring flights fOT the eval-
uation of calving. The initiation and pattern of elk
migration were regulated by snow cover. Snow depths
approaching 40 c~ (response depth) caused elk to move
to more exposed winter range, if such vas,~va11&ble.
Areas with DOre than '0 cm (critial depth) sno~ cove~
were avoided. South aspects and meadow cover types
.Mere used by elk ~re ~han other habitat types uuring
'the winter period. Estimates of winter range indicated
that a 15 percent increase in snowpack may decrease
total winter range by 0 to 20 percent dependent upon
pr~sent snow conditions. Spring movement of elk back
into the high summering areas was associated more
strohgly with the greening up of the open mountain
meado~ than directly with a receding snOW line.
Calving occured betveen mid-Hay and mid-J~ne at an
elevation of 2800 to 3200 meters. Each seaSOn there
appeared to be ample habitat available for calving
below the snow line.

INTRODUCTION

Justification

Elk (Cervus canadensis) represent a valoable natural
resou~ln 1968, hunters in Colorado spent over
$19 million on goods and services in connection with
elk hunting (Nobe and Gilbert 1970). 1he tremendous
esthetic appeal of the wild wapiti further increases
the value of this big game species. With such a pub-
lic investment involved, any action ~hat might affect
the elk populations should be scrutinized carefully.

In 1961, Congress directed the Bureau of Reclamation
to conduct research on weather modification, thus
initiating "Project Skywater". Within this nation-
wide weather m~dification research effort, the Color8-
do River Basin Pilot P~oject was established to test
fully the operational statu6 of winter cloud seeding.
The test site for this pilot project was a 8500 sq km
area of the San Juan Mountain Range of &outh~estern
Colorado. Much of the target area was above 2900 m
elevation. cooper. and Jolly (1969) cautioned that
"Although the snoW' management program contemplated
by the Bureau of Reclamation and other water sup-
ply agencies is initially to be directed mostly to-
W'ard high mountain zones of deep snoW' not used by
big game d~ring winter, spillover to lower eleva-
tion is not impossible. The effect on wildlife
populations might be significant. Perhaps more
important from the standpoint of the operating
agencies. the public may believe in such an effect
even if it does not exist. Wildlife is a subject
with such high emotio.nal content that sttong
efforts should be made to ensure that animal pop-
ulatio.1"ls are not adversltly affected by .....eather mod-
ification, and that the public is aware of this."

Secf'D"'S

C) ""', ,,<>_d

CONCLUSIONS

THe following conclu$ions were drawn for the elk
population on Missionary Ridge:
Hypothesis I - Critical Snow Depth
1. The initiation of ~inter nigration of elk is in
response to in~reasing snow COVer.
Z. SnoW' depths approaching 40 cm (response depth)
CAuse elk to mOve to more exposed winter range
if such is availab Ie. '
3. Snow depths in excess of 70 cm usually prohibit
the use of an area by elk, and is considered
the critical depth ot snow tor elk.
Hypothesis 2 - Aspect Preterence
4. .There may be a preferential use by elk of south
(~ aspec~s and/or mead,OW cover types during the
winter season.
Hypothesis 3 - Winter Range .
5. Winter ran e decreases ~th increasing snov
. cover. and may be decreased an average percent
by cloud seeding (15 percent 1.ncrease 1.n 8nO\l-
pack).
Hypothesis 4 - Spring Migration
6. S rin mt ration of elk is associated more
stron 1 with reening up 0 t e open moun Olin
meadows than directly w t t e rece ng snow
line. However, 1n late spr~ngs the receding
zone of partial snow cover may !lm~[ [0 some
degree the movement ot as much as ~u percen~ 01
the elk herd.
Hypothesis ~ - Elk ~lv1ng Location
7. ihe ma ority of elk calving oCCurs between mid-
Hav and mid-June at e evet onS or ~euu [0 jLU
8. The receding snow line in most cases does~ not
I' affect the location of calving.
Hypothesis 6 - Elk Cal~ng ~1tes
9. Each spring there appeared to be ample habitat
~or calvin~ below the ~one at par~lal snow cover.
10. Favored calvin& sites are characterized by a mix-
ture of timber cover and small openings with
southerly exposure.

y
"!of

In Steinhoff, H.W., and
~ountains, Colorado.
Present Address: Dept.

.T.D. Ives (Eds). 1976. Ecologi~al impacts of sno~?ack augmentation in the San Juan
San Juan Ecology Project, Final Report. Colorado State Univ. Publ.. Fort Cclli:1s.
of Forestry, Univ~rsity of Arkansas at MonticellO, Monticello. Arkansas 71655

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