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t . . E.n 1/ /o;rn-rM"\C.;TS .~.\1; CAL\'jt.;t, AS ?ELATEI. TO S~O'~' COYER J:il':.r!' :-:. s......t'rlf-V _~I <.nc H. \.,'. Stt:in!j("ff ABSTRACT A 4-year study \Jas conducted in the San Juan Moun- tai~ of south~estern Colorado to determlne t~-eTr€cts of snow on the distribution and calvinf; of elk-tCer\;lis- canadensis). Seventy-five a~rial st.:rveys resulted in 1262 \Jinter and 2325 spring elk sighting:::. l-!easllre- ments of the snowpack were obtained in creas or ~1nter elk slghtings and at 31 permanent snow stake sites to ~uantlfy the critical depth of snow for elk and to estimate winter range. Gr0und surveys provided supple- mental information to the spring flights for the eval- uation of calving. The initiation and pattern of elk migration were regulated by snov cover. Snov depths approaching 40 cm (response depth) caused elk to move to more exposed winter range, if such was',available. Areas with more than 70 c~ (critial depth) snow cover were avoided. South aspects and meadow cover types vere used by elk more thao other habitat types during the vinter 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 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 a receding snow line. Calving occured between 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 resource. In 1968, hunte~s in Colorado spent over $19 million on goods and services in connection ~ith elk hunting (No be and Gilbert 1970). The 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 night affect the elk populations should be scrutinized carefully. ;:n !')t;!, Cens.......';'; rH"'''('J'"nr1 ..1-,(' 'lI'1rn"l', "f Pn("~"".;:'-~"""': to conduct research on weather modification, thus initiating "Project Skywater". \,,'ithin this nation- wide weather modification research effort, the Colora- do RIver BasIn Pilot P~oject was established to test fully the operational status of winter cl0ud seeding. The test site for this pilot project was a 8500 sq km area of the San Juan Hountain Range of southwe~tern Colorado. Much of the target area ~as abov~ 2900 m elevation. Coope~ 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- ward high m?untain zoneS of deep snow not used by big game during winter, spillover to lo~er eleva- tion is not impossible. The effect Ol~ wildlife: populations might be significant.' Perhaps more important from the standpoint of the operating agencies, the public may believe in such dn eff~ct even if it does not exist. ~ildlife is a subj~ct with such high emotional content that strong efforts should be made to er.S\lre thdt anlr..al pop- ulations are not adverse1y affect~d by l.'e,1th(:l" TI;od- ification, and that the public is .'ll.'are of this." .See +'D"'~ C) "'. 1/<".& Cci~CLUS IONS THe following conclusions were drawn for the elk population on Missionary Ridge: Hypothesis 1 - Critical Snow Depth 1. The initiation of winter migration of elk is in response to increaSing soo.... COver. 2. Snow depths approaching 40 cm (response depth) cause elk to move to more exposed winter range if such is available. ' 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 aspects and/or meadow cover types during the I ~inter season. " Hypothesis 3 - \.'inter Range ( 5. ~inter ran e'decreases ....ith increasing SnOw cover, and may be decreased an average percenl \ by cloud seeding (15 percent Increase 1n snow- \ paCk). Hypothesis 4 - Spring Migration 6. S rin mi ration of elk is associated wore stron 1 with reening up 0 t e open wountain meadows than directly w t t e rec~ ng snow line. However, in late spr~ngs the recedIng zone of partial snow cover may Ilm~t to some degree the movement ot as much as iU percent ot the elk herd. Hypothesis ) - Llk Calving Location 7. The ma ority of elk calving occurs bet~een mid- May and mid-June at e evat10ns ot iCUU co ~LU 8. The receding snow line in most cases does~not I affect the location of calving. iHypothesis 6 - Elk Calving ~ltes 9. Each s ring there appeared to be ample habitat for calvin~ below t e zone 0 part a 6now cover. 10. Favor<<:~ ~l'yi?g~_si.te9 a~e characterized bv A m1x- ture of timber cover and small-openings with southerly exPosure. 1./ 3.1 In Steinhoff, H.~., and ~ountains, Colorado. Present Address: Dept. .T,D. lves (Eds). 1976. Ecolcgical impacts of sno\o..pack aug!:',er.tati.on in the San Juan San Jua.n Ecology Project, Finel Re.port. Colorado State Univ. Publ., Fort Ccl}.i:1s. of Forestry, Univ~rsity of Arkansas at Monticello, Monticello, Arkansas 71655 '15 A-8d-4