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o (~-:> c") C)1 -J l-" , Preliminary estimates by the U.S. Geological Survey indi4a~e that about 500,000 to 700,000 .cre....fee.t of ground wa_ter Illay be in stot'~ge on the mesa. The c:onforlDation of the m9untains basalt cap, coupled: with the iJaper'qLeable underiying fot'lD.at,~oQ.8 p:ro"ides <<.bowl shap.ed un'derground reserv9i~. Hiah precipitation, coupled with tne flat surface provides for significant recharge. llESCRIPTIO" OF THE AREA In 'sharp contrast to the alpine Grand Mesa are the low lying ari,d ".11eY8 t.o the south. west, and north. The Grand Valley lies to the northwest of the mesa along the Colorado River, and inc~udes the confluenae of that stream and the Gunnison Riv~r. To th.. south of the mes. are two connected ....alleys formed by the lower reach of the ,Gunnison River and its North Fork. A fourth val lay l~e. to che north of, th'e mesa along Plateau Cree'k, a tributary of the Colorado River. The Grand and Lower Gunni8on~ValleY8 are separated by a .crip of low hilla and'plate<<us, cut by relatively deep canyons, lying:west of Grand Mesa. The two largest communities in -tbe at'ea are the cities of Grand Junction in the Grand Valley ..nd Delts in the Gunnison Vall~y. Smaller communities include 'Fruita, Clifton, and Palisade in the 'Grand Valley; Cedaredge, Orchard Ciey. Hotchkiss. and paonia in the North Fork Valley; and Collbran snd Mesa in the Plateau Valley. The general stratigraphy of the srea is depicted in the sche'Dlati~"profile drawing sbown below.. It is a geologied] crosa-sect iOR from the Colorado Nstional Monullent on the Uncompahgre Plat'eau through the so~tbern portion of the Grand Valley to Grand Mesa. z 2 >- ~ " '" ~ '" COLO. NATIONAL (MONUMENT GRAND VALLEY DAKOTA MORRISON UMMEAVILL;E ENTRADA KAYEN'TA WINGATE "OLANO' AU\' T GUNNISON RI.....ER GRANITE CHINl. t SCHIST 4,000 Mesa-Delta M&I Gtound-Wa~er Study NoveJllliber 19-8J Pag<> 2 of 3 The basalt cap is 200-)00 feet thick. Gr8Ud Meaa. often refer~ed to 4S America's largest tabletop tQouneain, is un'ique bot'h aeol.ogically and -hydro- logically. It originated as << Se7:ie8 o,f lava flows issuing into ancient strea~ valleys. Subsequent erosion removed the softe~ sediment$ adjacent tQ the filled valleys leaving behind a fl,a1:-copped mountai~ with a resistant cap. The basalt. although quite porOU8 in some eases. has l~ttle pri~ary permeability sinc~ these pores have limited interconnection. Secondary fractures formed during cooling ot the molt~n lava and those formed by tectonic stresses. provide the permea- bility and storage spaces ,for' ground water. Ther~ are an estima_ted 300 small lake, and reser- voirs: on the top of Grand Mesa and on e,n- hummo,cky bench' below the, rila. They range in surface "rea ft"ol'll -less than one to 'nearly ,100 acres. The reservoirs, many ~f which are man~made enlargements of nacural lakeS, were eons,tructed to 8'<<:ore water for irrigators and municipalities in the ~alleY$. Water is stored in thes~ reservoirs during the spring; sno'Wmelt and released in lace seas,on when streara flow,s are ndttJt'ally low. The aJess is dr4in~d by many small streams floving in all direc.tians toward Plate'au Creek, the Gu-nnison River, or the ~orth Fork. COIl1lUCT .oF tHE stuDY The initia~ phases of the study would be 4irecte~ toward verification af an available ground-wa,ter s.ou-r,ce an Grand Mesa and e.valuation of, tlu!: needs for additional municipal. inetu-strial. and rural domestic water supplies in the t~o-CQUQty area. Drilling and testing wouJd be done to define the geologic and hydrologic cha.racteristics of the GRAND MESA BASALT UNNAMED UINTA 8 GReEN RIVER MANCOS