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ElK HO\T_'1E:~TS A!-u CAL\'It-\G AS RELATED TO S~O-,.; COVER }/ J3T:1f'!" ~. S\"'et'ne...]J anc H. \,t. StdnhClff ABSTRACT A 4-year study was conducted in the San Juan MQun- tal~ of southwestern Colorado to dcter~lne tne-eTTects of sno~ on the distribution and calving of elk~cer~ canadensis). Seventy-five aerial surveys resulted iii 1262 winter and 2325 spring elk sightings. Measure- ments of the snowpack ~ere obtained in areas of ~inter elk sightings and at 31 permanent snow stake sites to quantify the critical depth of snow for elk and to estimate winter range. Ground surveys provided supple- mental information to the spring flights for the eval- uation of calving. ~he initiation and pattern of elk migration were regulated by snow cover. Snow depths approaching 40 cm (response depth) caused elk to move to more exposed winter range. if such was'~vail&ble. Areas with more than 70 em C~ritial depth) snow cover were avoided, South aspects and meadow cover types were used by elk more than other habitat types during the winter period. Estimates of winter range indicated that a IS percent increase in snovpacK may decrease total winter range by 0 to 20 percent dependent upon present snow conditions. Spring movement of elk back into the high summering areas was associated more strongly with the greening up of the open mountain meadows than directly with B ~eceding snow line. Calving occured bet~en mid-May 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 valuable natural resou~ln 1968, hunters in Colorado spent over $19 million on goods and services in connection with elk hunting (Nobe Bod Gilbert 1970). The t~emendous 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 Skyvatey". \lithin this nation- wide weather modification research effort. the Colora- do'River Basin Pilot Project was established co test fuilY the operational status of winter cloud seeding. The test site for this pilot project was a 8500 sq km ayea of the San Juan Mountain Range of south"'e~t.ern Colorado. Much of the target ~rea was above 2900 m elevation. Coope~ and Jolly (1969) cautioned that "Although the 8n01ol management program contemplated by the Bureau of Reclamation and other water sup- ply agencies 1s !nitially to be directed mostly to- ward high mountain zones of deep snow not used by big game d~ring winter. spillover to lower eleva- tion is not impossible. ihe effect on vl1dlif~ populations might be significant.- Perhaps more important from the st.andpoint of the operating agencies, the public may believe in such an effect even if it does not exist. Wildlife is a subject with 6uch high emotional content that strong efforts should be made to enSure that ani~~l pop- ulations are not adverseq affected by "-eat her mod- ification. snd that the public is a-ware of this." - Secf,oY's D,.....; TI<?_d CONCLUSIONS THe following conclusions were dra~ for the elk population on Missionary Ridge: Hypothesis 1 - Critical Snow Depth 1. The initiation of Yinter migration of elk is in response to increasing snow cover. 2. ~Snow depths approaching ~O cm (response depth) cause elk to move to more exposed winter range if such is available. . 3. ~oov depths io 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 aspects and/or meadow cover types during the winter season. Hypothesis 3 - Winter Range \ S. Winter ran e decreases ~th increasing snow cover, and may be decreased an average percenl , by cloud seeding (lS percent increase 10 snow- pack) . Hypothesis 4 - Spring Migration 6. S rin mi ration of elk is associated more stron 1 with reening up 0 t e open mountain meadows than directly t t e rece ng snow line. However. in late .spr1ngs the recedIng zone of partial sno~ cover may !lmlC [0 60we degree the movement ot as much as LV percen[ dl the el.k herd. Hypothesis ~ - ~lk ~lving Location .' 7. The ma ority of elk calving occurs between mid- May and mid-June at e evat ons ot L~VU to ~LV 8. The recedin snow line in most cases does-not I affect the location of ca ving. ,Hypothesis 6 - Elk Calv~ng ~ltes . 9. Each s ring there appeared to be ample habitat ~or calvin below t e zone 0 pa~t a snow cover. 10. Yavor~d calvin sites are c aracter ~e yam x- ture o~~~j~mber cover and small openings wit souther~/ exposure. c' !I 1/ In Steinhoff. H.V., and ~ountains, Colorado. Present Address: Dept. J.D. Ives (Eds). 1976. Ecological impacts of snovpack 3ug1:lentation in'the San Jua!'1 San Juan Ecology Project. Final Report. Colorado State Univ. ?ubl.. Fo,~t. CclH':'.s. of Forestry, Univ~rsity of Arkansas at Monticello. Monticello, Arkansai~7l6SS -10- 415 A-8d-4