National Weather Service United States Department of Commerce

Mississippi Valley Summer Heat Wave; Severe Thunderstorms and Flooding Possible to the North

A dangerous summer heat wave will continue much of the Plains, Mid/Lower Mississippi Valley, Southeast U.S. today and Thursday. Areas of severe thunderstorms may occur today and again on Thursday from parts of the northern/central Rockies and High Plains into the Ohio Valley and Carolinas. Excessive rainfall over similar areas may bring flash flooding today and Thursday. Read More >

Graphic showing graph of weather and climate
Changes have been observed on almost all area rivers over the last several decades. Some of these changes are due to climate change and how it has impacted the water cycle. Humans may also impact hydrology in other ways, such as large scale changes to land cover.

Average Precipitation


Average precipitation differs across the area, generally from north to south. Yearly average precipitation (liquid equivalent) ranges from 35-40 inches per year in the southeast to 25-30 inches per year in the north. Yearly average snowfall is more variable, with 72-96 inches per year near the east shore of Lake Michigan, 36-48 inches per year west of Lake Michigan in Wisconsin, and as little as 18-24 inches per year to the south in central Illinois.

Graphic showing yearly average precipitation for the vicinity of northeast Illinois and northwest Indiana Graphic showing yearly average snowfall for the vicinity of northeast Illinois and northwest Indiana


The amount of rain or snow experienced in any given year is closely tied to weather. The average over a long period of time (most often 30 years) is climate. Precipitation in any given year can vary significantly from the yearly averages. The amount of precipitation is closedly tied to how much water ends up in area rivers. Long-term changes in precipitation can cause long-term changes in streamflow and water levels.


Weather vs. Climate


Weather and climate are tied together, but not the same! Weather is what one expects at a given location at a given time. Climate is the long-term average weather condition. Temperatures can average in the 50s in March (climate), but on any given day be 20F or 80F (weather). This also applies to precipitation.



Land Cover Changes


Factors other than precipitation can also impact the amount of runoff and streamflow. Rural areas tend to absorb more precipitation into the soil (infiltration) than urban areas. Forested areas also block precipitation from reaching the soil's surface, reducing runoff. Wetland areas (including swamps and marshes) can slow down runoff as it moves across the soil's surface to nearby streams. In contrast, urban areas tend to have fewer trees, less exposed soil, and more pavement. A higher portion of precipitation is turned to runoff in urban areas compared to rural areas.

Long-term changes in land cover can cause long-term changes in streamflow and water levels. Removal of forests and wetlands, straightening of streams, and new urban construction can all cause noticeable changes to river behavior including how sediment moves and how quickly streams rise. The construction of retention and detention ponds in urban areas can reduce some of the increase in runoff due to urbanization.

Notable land cover changes across our area include the widespread removal of forests and prairies for agriculture, the construction of large cities (such as Chicago and Rockford), and the removal of large wetlands (such as the Grand Kankakee Marsh, the Inlet Swamp, the Winnebago Swamp, and the Vermilion Swamp).


Precipitation Trends


Over the last several decades, average weather patterns across area river basins have changed. These climate changes have included not just increases in average temperature, but also increases in average precipitation. For example, in the northeast Illinois climate region, average annual precipitation was about 34 inches (liquid equivalent) back in the 1950s but climbed to 40 inches in the 2010s. An increase in average precipitation has been observed across the entire area, about 10-25%.

Graph showing annual rainfall and trend for the northeast Illinois climate division Graph showing rainfall trends fpr climate divisions covering all area river basins

This increase in precipitation does not mean that precipitation in any given year will always be above average - precipitation will continue to vary from year to year. These changes do mean, however, that the chance of a very dry year has decreased, and the chance of a very wet year has increased. This increase in precipitation has implications for the water cycle of area river basins.


Streamflow Trends


As with weather, there are typical ranges for water level at a given location throughout the year. In general, the long-term average water level is controlled by the water cycle – especially the precipitation and evapotranspiration within a river basin.

Changes in precipitation have led to significant changes in runoff into area rivers, although not all area rivers are changing at the same rate. For example, the average streamflow in the Rock River (measured at Joslin, IL) has increased from near 5500 cfs in the 1950s to near 11000 cfs in the 2010s, a 100% increase. Flow in the Kankakee River (measured at Wilmington, IL) has only increased by about 40% over the same period.

Graph of streamflow trends in the Rock River Graph of streamflow trends in the Kankakee River

Due to this increase in streamflow being passed down area rivers, average water levels have increased. This means that the chance of reaching flood stage in any given year has increased for most areas. Due to a combination of urbanization and increases in precipitation, river basins in the Chicago Metropolitan Area have seen some of the largest increases in streamflow.

Changes to the Water Cycle


How can a small increase in precipitation (10-20%) cause such a large increase in streamflow (50-100%)? It is related to the complicated way that climate changes are impacting the water cycle. The various parts of the water cycle are changing - or not changing - in different ways and at different speeds.

In the Rock River Basin, for example (below), precipitation has increased from about 32.5 inches a year in the 1950s to just over 39.0 inches per year in the 2010s, while evapotranspiration has stayed close to the same at around 24.5 inches per year. This has caused runoff to increase from about 8.0 inches per year in the 1950s to about 14.5 inches per year in the 2010s. The 20% increase in precipitation has caused runoff to almost double (more than an 80% increase).

Water Cycle Changes: Rock River Basin
In the Rock River Basin (upstream of the outlet near the Quad Cities), precipitation has increased about 6.5 inches per year since the 1950s, while evapotranspiration (evaporation + transpiration) has stayed about the same. This has resulted in a significant increase in runoff.
Precipitation 32.5 inches 39.0 inches
24.5 inches 24.5 inches

Runoff 8.0 inches 14.5 inches
Runoff is directly comparable to streamflow (and average river levels) in the water cycle. This change has been confirmed by measurements of streamflow taken in the Rock River (near the basin outlet), which have also nearly doubled over the same time period.


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