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Dredqinq. This alternative consists of physically removing the island from the channel. This would require excavation of the material to an elevation low enough to allow the river to flow over the top of the remaining sandbar at least 50 percent of the time. This inundation would reduce the rate of buildup and decrease maintenance cost. The geomorphic analysis indicates it takes 8 to 15 years for emergent sandbars to become fully vegetated islands. The amount of material that would need to be removed is 166,600 cubic yards, and a total of 13.8 acres would have to b.~ cleared. Eventually, the island would most lik'~ly reestablish itself and the material would need to be removed again. The expected life of this option is 10 years. The life of this alternative could be extended by periodic clearing of the vegetation on the emergent areas of the sandbar. This could be accomplished by mechanical or chemical means. The lack of vegetation will slm. the rate of sediment deposition and extend the life of t:he project to 15 or 20 years. The advantage of this alternative is immediate removal of the island. The major disadvantages arE' continuous high maintenance costs and possible adverse environmental impacts. A plan view of this alternative is provided on Plat"e 12, and a comparative cost estimate is presented in the next s,ection. Channel NarrowinC(. This alternative consists of constructing transverse dikes along the right and left banks upstream from the bridge. These dikes 1f/ould constrict the river to a top width of 600 feet. It is desirable to have t:hE' fully constructed channel in the center of the bridge; therefor<~1 'the dikes would need to be constructed in stages due to the narrowness of the channel along the right bank. As the island begins to erode, 'the dikes would be extended until the top width is 600 feet,. Each dike would be constructed to an elevation 2 feet below 1:he high bank, with the crown of each dike remaining constant over 'I:he entire lem,th. This should, with time, erode most of t:he island and create a self- maintaining channel through the bridge. Because of the heavy vegetation on the island, some clearing and/or dredging of the island may be necessary to completely remove the island from the bridge area. The major advantage to this alternative is lower average annual maintenance. The disadvantages include a lengthy construction period and possible adverse environmental impacts. A plan view and typical sections are presented on Plates 13 and 14, and a comparative cost estimate is presented in the next: section. Sect.ion 22 - South Platte River, Mer:.no, Colorado Page 4