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and R57-R58. Fi ure 4B shows article size distribution lots for sam les collected &om lines g P P P R59-R60, R61-R62, and R63-R64. Both of the figures show samples collected from the top surface of the accumulated sediments in black and sam les collected from core sam les in red P P with the same plotting symbol as the corresponding surficial sample. In these plots, a downward shift in the position of a line indicates an increase in the particle size ' of the materials. On a sam le to sam le basis the surficial sam les do not show the usual P P � P tendency for sediment particle size to decrease from upstream to downstream in the lake, a result of the natural sorting of suspended sediments in the lake environment. Coarse sediments are deposited as the inflowing stream water is first slowed upon entering the lake. As the water moves through the lake, the suspended sediments become finer as the coarser sized fractions fall out of suspension. This trend is apparent overall in the significantly higher sand proportions in figure 4B than in figure 4A. The e}cistence of a major sand portion in these samples is also unusual for Illinois lake sediments. Both of these unusual observations may result from the low trap e�ciency and large streamflow volumes that apply to this basin. The particle size distributions of figure 4 do show the common tendency for sediments at any point in the lake to become more coarse with time. The more recent, surficial sediments (shown in black) that have been deposited on top of older sediments (shown in red) are characteristicalIy much more coarse. This observation is consistent with general trends in lake sediments. Surficial sediment, the most recently deposited sediment, tends to be coarser with time at a given point. This is due to the downlake shift in the initial depositional environment of the lake due to the loss of trap efficiency of the upper end of the lake. With time, the initial depositional zone in the lake will move further down the Iake because of water volume loss to sedimentation. Summary The Illinois State Water Survey has conducted a sedimentation survey of Basin 6 of Lake Decatur in Decatur, Illinois. The lake, originally constructed in 1922, is the raw water source for the Decatur public water supply. Previous lake sedimentation surveys were conducted in 1936, 1946, 1956, 1966, and 1983. The operating elevation for the reservoir is 614.4 feet NGVD during the summer months and 612.5 feet NGVD for the winter months. Sed'unentation has reduced the capacity of Basin 6 of Lake Decatur from 2,797 ac-ft (911 million ', gallons) in 1922 to 1,451 ac-ft (473 million gallons) in 2000. The sediment accumulation rates in ' the lake have averaged 17.3 ac-ft per year from 1922 to 2000. Annual sedimentation rates for six separate periods, 1922-1936, 1936-1946, 1946-1956, 19�6-1966, 1966-1983, and 1983-2000, I were 3�.4, 25.3, 10.9, 17.1, 10.4, and 8.3 ac-ft, respectively. 13