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00619 The low dams \vou I d be concrete grav I ty structur'es Cine! des i gned to pass floods greater than the des i gn flood over the -1-01' without damage. In the caseof Strontla Springs, the period of overi'opplnq would I ast for about five hours and wou I d approach nine feet in depth. Adequate structural measures to withstand this overtoppIng are r'equI red. For emergency draw-down purposes, low-level conduits would be desirable in each of the high dam alternatives. SEDIMENTATION The stream bed in Wa.terton Canyon j s composed of cobb I es and boulders which are overlain by Intermittent bars of sand and gravel that migrate downstream through the reach. During low flows the water is re I at i ve I y c I ear, but when snow me It causes the channe I 'ra run deeper, the sands are In suspension. Following mountain rainstorMs, the stream flow Is muddied by sl Its and clays that are washed down by the ralnfal I runoff . Sma II dams, 50 feet or less in he I ght 'II I th the capac I 'ry to store only one or two years of average sediment load, have only a local effect on the stream channe I reg i men. After a few months of operert I on, the incoming sediment load would be sluiced through the dam and would be redeposited downstream. Higher dams wIth large sediment storage capacities of about 100 years wou I d affect the downstream channe I reg I men by i nter'cept I ng the sand and gravel load. ThIs would result in eventual removal of the sand and gravel bars from the downstream channel and a consequent reduc- tIon in suspended sand concentrations. Extremely r'are flood events, similar to the Big Thompson, could produce extraord I na ry sed I ment loads that wou I d dr'ast I ca I I Y ,educe i'he storage capacity of the large In-canyon reservoirs. STREAM WATER 9UALITY The water of the South Platte River flowing through \'iaterton Canyon I s of very high qua I i ty. Placement of f I I I mater i a I In i'he river for dam construction would permanently eliminate benthic (bo'j.toro- dwelling) communities and aquatic habitat at the darn si'res. The pool created by any dam would permanently convert a segment of stream channe I into I ake and, in that segment, wou I d a I tei' the benth i c and b 101 og I ca I commun I ties of a fr'ee-f low I ng str-eam into 'rhose assoc I aj'ed with an impoundment. Construction activities would tempo,a,lly Increase 12