Since 2015, the U.S. Geological Survey, with financial assistance provided by Illinois Environmental Protection Agency, has operated a network comprised of “super gages” that provide real-time continuous monitoring of eight major rivers in the state (Figure 3.9 and Table 3.5). Each super gage continuously measures river discharge, along with nutrient and suspended-sediment concentrations to calculate continuous loadings. Nutrient and suspended-sediment loads were computed for the 2016- 20 water year period using a statistical modeling approach. These load estimates represent the riverine loads at the locations of the eight monitoring stations and include parts of the Illinois, Rock, and Vermilion River basins that receive drainage from neighboring states. The loads do not include an estimate of ungaged areas of the state. The cumulative drainage area of these gaging stations covers approximately 74% of the total land area of Illinois, as well as portions of Wisconsin and Indiana. The statistical modeling procedure provides a means to both determine and reduce uncertainties in load estimates computed from continuous data with data gaps. The load estimates are currently under review and awaiting approval; a report describing the modeling procedures and results is scheduled for publication in 2021. A detailed comparison of load estimates using this procedure and load estimates computed from discrete water quality data is forthcoming.
For the 2016-20 water year period, the provisional mean annual nitrate-N load was 426 million pounds N per year; the provisional mean annual total phosphorus load was 30.2 million pounds per year; and the provisional mean annual suspended-sediment load was 8.32 million tons per year.
Drainage Basins and Monitoring Stations
Figure 3.9. Drainage basins and locations of the eight nutrient monitoring stations
Table 3.5. Monitoring station information
River | USGS ID | Station Drainage Area
(km2) | Station Drainage
Area in Illinois (km2) | Basin Drainage
Area in Illinois (km2) | Percent of Station Drainage
Area in Illinois | Areal Percent of Illinois |
Vermilion | 03339000 | 3,341 | 3,105 | 3,372 | 93% | 2.1% |
Embarras | 03346500 | 6,042 | 6,042 | 6,307 | 100% | 4.2% |
Little Wabash1 | 03381495 | 7,998 | 7,998 | 8,298 | 100% | 5.5% |
Rock | 05446500 | 24,732 | 10,290 | 13,789 | 42% | 7.1% |
Green2 | 05447500 | 2,598 | 2,598 | 2,927 | 100% | 1.8% |
Illinois3 | 05586300 | 69,264 | 58,666 | 64,009 | 84% | 40.2% |
Kaskaskia | 05595000 | 13,439 | 13,439 | 15,045 | 100% | 9.2% |
Big Muddy | 05599490 | 5,592 | 5,592 | 6,180 | 100% | 3.8% |
1 Drainage area numbers are for Little Wabash River at Carmi, IL (USGS ID 03381500).
2 The Green River is part of the Rock River drainage basin. The 2,927 km2 is part of the 13,789 km2 Rock River drainage area in Illinois.
3 Drainage area numbers are for the Illinois River at Valley City, IL (USGS ID 05586100).
