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• <br />PARSONS <br />• B. Questions Regarding Sediment Transport <br />• C. Questions Regarding Vegetative Encroachment <br />• D. Questions Regarding Macro - Historic Natural Variability <br />• Geomorphic and Vegetative Consequences of "Proposed Program" <br />This Technical Memorandum describes the results of Investigation Task Al, the first of <br />five tasks comprising Issue Category "A" — Questions Regarding Channel Narrowing or <br />Deepening. <br />Presumed Pre - Development Conditions of Platte River System <br />According to the authors of the draft report entitled "Platte River Channel: History <br />and Restoration" (Murphy and Randle, 2001a), prior to the development of water <br />resources within the Platte River basin (which largely occurred during the 1900s), the <br />Platte River channel was wide, shallow, and braided in geomorphic configuration. <br />Annual flood peaks were high — commonly greater than 10,000 cubic feet per second <br />(cfs), sediment loads were large, and the median grain size of the river bed was fine sand. <br />Overall channel widths including active (flowing) and inactive (dry) channels, and <br />intervening braid -bar systems, were on the order of one mile, and the shifting sand bars of <br />the braided river system kept the channel relatively free of vegetation. As stated in the <br />draft report entitled "Platte River Channel: History and Restoration" <br />"During the period 1902 -1909, the average annual peak flow (average annual <br />maximum of mean daily flows) at the stream gage near Overton, Nebraska was 20,500 <br />ft3 /s, and the mean annual flow rate was 2,900 ft3 /s. (Murphy and Randle, 2001a, p.2) <br />"The Central Platte River before the 1900s was dynamic, changing from year to year, <br />and may have been slightly aggrading in certain reaches. The river is nearly straight, <br />with a channel slope equal to the valley slope.... Overall, the channel was likely in a <br />natural state near dynamic equilibrium ... (Murphy and Randle, 2001a, p.2) <br />"Floods and droughts would come and go and the river would change in response to <br />these flow changes, but the channel effects of these flow variations would fluctuate <br />around average channel parameters that described the long -term- average properties <br />of the river." (Murphy and Randle, 2001a, p. 3) <br />During the 1930s and 1940s, large storage reservoirs were constructed in the Platte <br />River watershed to provide water for irrigation. These reservoirs were (and are) used to <br />store water during periods of high streamflow, for release during later periods of low <br />streamflow, or as needed. The authors of the draft report entitled "Platte River Sediment <br />Transport and Riparian Vegetation Model" (Murphy and Randle, 2001b) claim that this <br />pattern of reservoir storage: <br />"significantly reduced the annual peak flows and the application of water to <br />agricultural lands reduced annual flows within the river channel. The large storage <br />reservoirs also trapped the sediment load of the North Platte River and significantly <br />SAES \WP\PR0JECTSl3- States\A 1 Final Tech Memo.doc <br />-2- <br />