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<br />000167 <br /> <br />CHAPTER l. DESCRIPTION OF V ALl.EY <br /> <br />PHYBICAL FEA Tt.'RES <br /> <br />The Sail Luis \" alley (hereinafter .e{{'rred to &:i the nlley) is located <br />in 8outh-central Colora.do and has a drainage area of about 8,000 <br />square miles. The northern portion of the valley, which constitutes <br />the Closed Basin. has a dralUable area of about 2,940 square miles <br />which is s(I'plLrated at the southern end (rom the Rio Grande watershed <br />bv ala\\" allU\;al dh;cle. The floor of the vallev. which lil'S at a general <br />efevation of about 7,500 (eet abo"e mean sea. level. is an almost tree- <br />less plain which extends from its northern end, at Ponc.ha Pass, about <br />90 mil(>S south t{l the Colorado-New ~Iexico State line. It varies in <br />width from about 20 to 50 mil{'s as shown on the General map, <br />drawing 253-504-1759. . <br />The San Juan :\Iountaills Bnd the Continental Dh;de (orm the <br />wcst boundary o( the nlley and the SILngre de Cristo Range (orms the <br />('ast boundar)"" The mountaiIL_" are generally heavily wooded. and in- <br />dude more than II. dozf'n peaks at elevation~ in excess of 14,000 (eet. <br />~fany streams flow Crom the mountainous areas into or across the <br />valle)". The Rio Grande Ilnd tbe Conejos River are the largest o( these <br />streams. The (ormer cuts es.;;;entiall,y across the valle, (rom west to <br />east before it turllil south nf'ar Alamosa. The COlleJos River flows <br />generally along th\" southern boundary of the valley to enter t.he Rio <br />Grande n('ar La ~auses, <br />The land!:> on the vallf'Y floor art" largely devoted to irrigated agri- <br />('ulture, Those not uSI~l for irrigation, excluding ext.ensive sand dune <br />f!.r('as, arf' covf'rNI with salt grass, gTt'asewood, rabbit brush, and other <br />walf'r-consuming v{'getation and are used (or grazing livestock. <br /> <br />CLIMATE <br /> <br />Tlw '...allf'Y is II. high mountain desert with an IlVf'rage annual pre- <br />cipitlltion or about 7 incht's on the valley floor. In the surrounding <br />mountain are-as, the aVt'rIlge annual prl'cipitation increa.<;es with <br />a1titudt> up to about 30 im'hes, <br />Summ('r days are warm, bllt nights are cool with an 8"erag-t' daily <br />spread of about 3.50 F. In wint-t'r the wmpt'raturp is low much of the <br />tmle, and in most years a low reading of about minus 300 F. is ex- <br />(>l'rienn.{1. G(,llemlly, there is littl{' snow on thl' valley floor, and the <br /><lftVS are quite com(ortabl{' bt'caust. of the large amount of solar <br />raJiatioll. <br />Th(' growinj! sea.~n in the valley is short, ranging from 90 to 120 <br />days. <br /> <br />GEOLOGY <br /> <br />The San Luis Valley is 1\ structural b/l.'lin bordrrt'd bv faults on the <br />t'lIst and pl?rhaps also '011 tht' wt'St, althou~h the structural relations 011 <br />th{> west are Jargt'!y obscured by tl?rtiary ,"olcallics, <br /> <br />(5.".iJ <br />