. The changing Illinois environment : critical trends : technical report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. COVERAGE (HA) 1988 YEAR. Sag 2 Sag 3 Seg4 Sags Sag 6 Segment Figure 47. Vegetation cover in the lower Des Plaines River (river miles 273-286) during the past seven years. About 65% of the total 693 ha of water surface area, or 450 ha, is potentially habitable by submersed and emersed vegetation. This includes main channel border, slough, and side channel areas but excludes the main navigation channe
. The changing Illinois environment : critical trends : technical report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. COVERAGE (HA) 1988 YEAR. Sag 2 Sag 3 Seg4 Sags Sag 6 Segment Figure 47. Vegetation cover in the lower Des Plaines River (river miles 273-286) during the past seven years. About 65% of the total 693 ha of water surface area, or 450 ha, is potentially habitable by submersed and emersed vegetation. This includes main channel border, slough, and side channel areas but excludes the main navigation channel. During earlier surveys 10-13% of the 450 ha was vegetated; in 1991 the vegetated areas constituted 6% of the potentially habitable areas. Peak vegetation levels (60 ha) were reached in 1987, and since then there has been a steady decline to the estimated ha in 1991. The above graph demonstrates the significant decline in total vegetation cover in the lower Des Plaines River from 1985 to 1991. There are a number of hypotheses to explain the decrease in plant populations in the study reach, most relating to physical factors. The drought of 1988, and carryover of those conditions to a lesser degree in 1989, may explain some of the decline in coverage. Decreased water depths both during the drought and afterward, resulting from increased sediment deposition due to poor watershed manage- ment, may account for some of the most significant decreases in vegetation cover. With decreases in water depth it is likely that much of this area became too shallow to support submersed vegetation. Drought conditions in 1988 may have caused barge traffic to increase. Low water levels may have necessitated an increase in the total number of barges handling smaller individual tows to avoid running aground with more traditional larger tows. Under this scenario, the frequency of disturbance would increase although the magnitude would decrease, which may affect aquatic vegatation more severely.
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