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-8- I I I I I I I I I I I I I I I I I I I ~ports, and extracting and processing energy producing ~inerals 1S expected, and pressure leading to intensified flood plain use WIll undoubtly accompany such development. At prescnt, some residentia~ development in the GlenwQod Springs area ap- pears to be takIng place without consideration of potential flood hazards. The climate of the upper Colorado River basin and Roaring Fork River basin can be characterized as semiarid except in the higher elevations where precipitation is moderately heavy and winters arc severe. Pronounced climatic variations occur within relatively short distances due to the varying topo- graphy. Study Reden Description T~c Roaring Fork Ri~cr study reach extends from the confluence wlth the Colorado Rlver south of the City of Glenwood Springs along Highway 82 7 miles to the area of Cattle Creek. The Colorado River study reach extends fron the 'West Glenwood Springs Interchange on 1-70 east some 2 ~ilcs to the con- fluence with the Roaring Pork River. The Index Map, Plate 2, shows the study li~its, The,normal channel for the Roaring Fork River cuts through a wlde valley with a relatively deep channel, SUpported by farmland above. In recent years develop~ent pressure has in- creased along portions of low channel ~rcas of the river. Development is extensive northwest of Glenwood Springs along the Colorado River. Little development as of yet has Occurred along the south bank. Obstructions to flood flows within the study reach include natural obstructions such as brush and other vegetation growing al?ng.the streamb~nk and man-made obstructions such as bridges, bUlldlngs and trallcrs. During floods these obstructions im- pede flood flows and cause backwater conditions that may in- crease the flood heights upstream of the obstruction and in- creased velocities downstream of the obstruction. Brush wasn~d out durlng t~e floods carries downstream and may collect at bridges and ln culverts, thus r.re~tin9 a dam~ing effect and overbank flow. t\S Elood flows increase, masses of debris may break loose and cause a wall of water and debris to surge downstream untll another obstn.:ction is encountered. In some instances, debris may Collect to the point where structural capability is exceeded and a bridge is destroyed or abutments and approaches eroded and roadbeds and railroad beds damaged. There are no existing or authorized flood control structures in the study area and none are under investigation. Characteristically, great temperature changes occur from summer to winter, and local weather conditions may change rapidly due to movement of storm systems from w7st to cast through the region. Precipitation is directly lnfluenced by elevation with the river valleys receiving 10 to 15 inches and the high mountainous northeastern are~ receiv~n~ 30.to 40 or more inches annually. Most wintertlme preclpltatlon occurs as snow and a deep snowpack normally accumulates. In the study area, average snowfall is about 70 inches at . Glenwood Springs. Generally, a few large snowstorms dom1nate seasonal snowfall. Snowpack normally begins to accumulate in late October and snowmelt begins in late April and con- tinues into June or early July. General rain may occur ov:r large arcas from late spring through late fall and conv~etlve type cloudburst storms occur in summe~. Aver~ge monthly precipitation varies between I and 2 lnches wlth t~e largest amounts usu~lly occurring f~om January through Aprll and ~n August. In river valley a~eas, mean maximum temperature. varies from the mid 30's in January to around 90 degre~s ln July, while mean mini~ums vary from ar?und l~ d~grees ln January to 50 degrees in July. Mountaln reg~ons are cooler. Record low and high readings in Glenwood Sprlngs are -30 de- grees to 55 d~gr~es in January and 38 degrees to 100 de- groes in July. Vegetation varies from sagebrush and as~ociatcd semia~id growth to al?ine and tundra type cover ln high mountaln area:;. Agr icultural ope~aLions dud, L.lrb'"nization. hav7 d] .,: i.c_llly ~odified natlve vcgetatlon ~n the maln rl~~r v~llcys, the adjoining bench lands , and Lh~ low~r port~o~s o( sl..lll tributary valleys. Bluffs and hllls not modlfled by "g' ]o;:"lture ,lre characterized by j,:,ni?~r nnd pin?" for- est covor with onk brush and sage dom~natlng the drler slopes, Lands above 7,500 feet have a Characteristic cover to srJ"uce, fir,lodgc;:>ole pine, and aspen with t~e t~ee pat- tel.I'.: "tro;lgly influenced by exposure. Vegetatlon ln the high~~t mountain areas is classified as alpine slope and meadow with cover of mixed alplne tundra grass, sedge, forbs, and alpin~ willow. The flood plains of the streams under study arc not presently exte~sively developed. There arc some in~ustria~ ~n~ rural rc~idcntial uscs and various lransportdtlon faclI1tles are in flood plain a;eas, but the predominate use is aqricul- tunl-l. However, continuing emphasis on tourism, winter . -9-