Streamflow Characteristics

To characterize water availability in streams, rivers, and lakes, long-term continuous streamflow gages provide critical streamflow data which can be used to analyze magnitude, frequency, duration, timing, and rate of change of streamflow (Liu et al., 2018). There are 10 USGS streamflow gages in the Lower Illinois WSPR.

The periods of record of the gages range from 1 to 101 years and they monitor a wide variability in drainage areas, from headwaters to large watersheds ranging from 2.4 to 26,743 square miles (sq mi). 

Two gages (Illinois River at Valley City (USGS 05586100) and Macoupin Creek near Kane (USGS 05587000)) are active long-term gages. The gage at Valley City, which covers a drainage area of 26,743 sq mi, began operation in 1938 and has been operating ever since. The gage at Macoupin Creek near Kane, which covers a drainage area of 868 sq mi, began operation in 1921 and is still in operation 101 years later. However, the gage was not operated for the period 1934-1941.

The other 8 gages are currently inactive, with periods of record ranging from 1 to 51 years with an average of 18 years. Four of the gages monitored headwaters or small streams with drainage areas of 100 sq mi or less, and only one of them is along the Illinois River near Meredosia (05585500); it had a drainage area of 26,029 sq mi. Nevertheless, the streamflow records of these gages can be used to estimate streamflow statistics using hydrology extension techniques such as the index gages method (Zhang, 2017) or the baseflow correlation method (Zhang and Kroll, 2007).

As different gages have varying periods of records, the streamflow records of the gages are not comparable if they are obtained from different periods. In addition, hydrology analyses based on short-term streamflow records are often not representative of long-term conditions. For regional water supply studies, representative streamflow records are needed to cover periods of wet, normal, and dry conditions. The selected period for regional hydrology analysis is termed the base period. Based on the common period of records of the two long-term gages, the period of 1941-2022 was selected as the base period.

Flow duration curves (FDC) graphically show the variability of streamflow and demonstrate the relationship between the magnitude and frequency of streamflow (Zhang, 2017). The variability and thus the shape of FDC are impacted by watershed characteristics, especially drainage areas.

The FDC for the gage on the Illinois River at Valley City is much flatter than that for the gage on Macoupin Creek near Kane, demonstrating that the streamflow in the Illinois River is more stable than in Macoupin Creek. For example, the flow at the Valley City gage ranges from 1,330 to 123,000 cubic feet per second (cfs), while the flow at the Kane gage ranges from 0 to 38,100 cfs. The ratios of the maximum flow to the mean flow are 5 and 66 for the Valley City and Kane gages, respectively. From the perspective of water supply, the Illinois River not only provides more water but also is more reliable with less variability.

Surface water availability is often determined by low flow or drought conditions and is generally characterized by low flow statistics. To describe low flow conditions over a varying period of time, low flow statistics are often estimated based on streamflow records using a moving period window, usually 1 to 90 days. The annual minimum 7-day flow for the gage on the Illinois River at Valley City is used to demonstrate the low flow trend and fluctuations in the Illinois River.

The mean 7-day flows for periods pre- and post-1970 were estimated and shown in the Figure above. The annual minimum 7-day flow increased over a short period around 1970, being statistically significant at a 99% confidence level. An abrupt increase in low flow has also been observed for rivers and streams in Illinois and other parts of the U.S. (Kelly et al., 2018; Knapp, 2005; Zhang et al., 2010). The variability of the annual minimum 7-day flow appears to have increased since 1970, but the mean of the annual minimum 7-day flow appears to be stable, which indicates that the annual minimum 7-day flow has remained at the increased level. The lowest annual minimum 7-day flows were observed in 1962–1965, 2005, and 1956.

The gage on the Illinois River at Valley City represents the cumulative hydrologic conditions upstream of the gage, which cover a vast drainage area and may not be representative of local conditions in the Lower Illinois watershed. The long-term gage at Macoupin Creek near Kane is used to explore low flow trends in local watersheds.

The mean annual minimum 7-day flow prior to 1970 was 10 cfs, and it increased to 15 cfs for the period after 1970. The increase is statistically significant at a 90% confidence level. The increase also appears to have happened in a relatively short period around 1970, as observed for the Valley City gage. The increased low flow in the Lower Illinois watershed implies that the potential available surface water supply has increased. However, it also means that the corresponding minimum flow requirement will be higher as well. It is not clear if the increased low flow will change in the future. As surface water supply systems in the Lower Illinois watershed rely on water supply reservoirs, increased low flow would increase yield from these reservoirs, excluding the impact of losing reservoir storage capacity due to sedimentation.

This story map is part of a series about Water Supply Planning throughout the state of Illinois. Water Supply Planning is an ongoing research effort conducted by the Illinois State Water Survey and is funded by the   Illinois Department of Natural Resources Office of Water Resources.