| OCR Text |
Show 12-year period (1930-41), 15.4 million acre- feet. 7-year period (1942-48), 26.9 million acre-feet. The annual average depletions for irrigation and associated uses in the upper basin are esti- mated at something over 7 million acre-feet after completion of the presently contemplated pro- gram. This is not expected for over half a century. Estimates of depleted flows below Yankton must take into account the supply of water taken from storage in the reservoirs. The average depleted and regulated flow at Sioux City, Iowa, from 1898 to 1948 is estimated to be 15.7 million acre-feet annually. This figure takes full account of all irrigation. The present authorized navigation project was designed for a flow of from 25,000 to 30,000 second- feet during the navigation season with a minimum of maintenance dredging. This is about 15 million acre-feet a year, or slightly less than the estimated regulated flow available at Sioux City. However, on the basis of the available data, 15 million acre- feet cannot yet be accepted as a certainty during an extended dry period like that of 1930-41. The re- quirements of the various agricultural measures are not yet known, nor are the actual amounts of carry- over likely to be at hand at the beginning of a drought. Furthermore, the extent of possible in- dustrial uses in the middle and upper basin is not known. The compatibility of navigation therefore must be examined in the light of possible adjustments in the navigation plan which will reduce its flow requirements. Such adjustments are possible through increased channel maintenance; short- ened navigation seasons; refinement of bank stabili- zation control works; and slack-water navigation. However, none of these measures may be required during at least the next 30 years, because full de- velopment of irrigation possibilities may require 50 years, and completion of farm conservation meas- ures 30 years. Regulated flows at Sioux City during this period are likely to average considerably above the minimum required for open channel navigation. If reduction in flow eventually is required by upstream uses, increased channel maintenance may be the first measure employed. This would pre- scribe maintenance of the 9-foot depths at critical locations by dredging rather than dependence on natural scour. A second measure, employable in critical intermittent years, could be a shortening of the navigation season, and therefore a reduction 200 in water releases for part ozf the normal navigation season.6 A third step would be refinement of bank sta- bilization control works. Rurther shaping at some points would permit maintenance of required depths by natural scour. A final possibility is modification of the project to provide slack water navigation by constructing a series of locks and dams. This may become prac- ticable with control of floods and a reduction of sediment.6 Such a project, from the point of view of navigation, would requrre only sufficient water for seepage and for lockages. With slack water pools, it would be necessary to stabilize the banks as is now being done under the navigation project. Therefore, the Corps of Exigineers considers that the work now being done would also be useful and a necessary part of a slack water navigation project. Slack water navigation, in view of the minimum releases necessary for other purposes on the lower river, would certainly not conflict with upstream requirements. Conclusions The principal possible sources of conflict of navi- gation and other water uses is with irrigation re- quirements. The probability of such conflicts within the next 30 years is not high. Beyond that period possibility of conflict is minimized by meas- ures which could be taken to reduce water flow requirements for navigation. Water requirements for navigation would reach a minimum in a slack water channel. However, this would depend on prior channel stabilization and solution of the sedi- ment problem and other attendant uncertainties. Studies are continuing on adjustment of naviga- tion to water requirements for other uses. It is therefore concluded on the basis of available data that navigation does not appear to be physically in- compatible with other water uses on the Missouri. If a slack water channel is needed, navigational adjustments may require large expenditures, which will raise questions of economic feasibility. Those questions and questions of tolls have not been con- sidered here. 8 Shortened seasons, however, would add to unit costs, and therefore influence feasibility. 6 Certain other problems, like seepage into valley bottom farm lands, also remain for solution. The low flow also might create difficult problems of pollution control. |
