Earthshots: Satellite Images of Environmental Change

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These three Landsat 5 images show several rivers in southern Illinois and Indiana, USA, during normal and flooded conditions. Peak flooding was observed in this area on June 10 and 11, 2008. The clear Landsat scene from June 11, 2008, and the clear image of “normal” conditions from June 9, 2007, allow us to compare the scenes and see exactly what damage a flood of this magnitude can cause.

Rainfall amounts ranging from about 2 inches to more than 10 inches fell in this area on June 6–7, 2008. Spring was wetter than normal, so the heavy rain easily saturated the ground. The rivers quickly rose to exceed flood stage.

Satellite images can help authorities respond to disasters such as floods. The images can help local authorities see the amount of flooding and where there is damage to property. The extent of the flood can be mapped so that response teams can view where they are needed and respond quickly.

Map of the featured area.

The flooding that happened in summer 2008 in Indiana and Illinois started late in 2007 with above normal snowfall. This extra snow saturated the ground as it melted in spring 2008. The above normal rainfall in the spring only made the situation worse. (Many U.S. Geological Survey streamgages in the region already showed stream levels at higher than average streamflow.) The heavy rainfall event on June 6–7, while by itself might not have caused catastrophic flooding, combined with the saturated conditions from the previous winter snows to cause this flood.

On June 6, 2008, a nearly stationary (unmoving) weather front was draped across south-central Indiana, and moisture from the Gulf of Mexico streamed north to fuel thunderstorm development. Nearly continuous thunderstorms over a 12–16 hour interval dumped significant amounts of rain on the region. This rain flowed into the already high rivers and streams, which rose quickly.

In the precipitation map on the left, the colors indicate the amount of rainfall for the first half of June 2008. The colorful area from eastern Illinois and into central Indiana reflects the heavy rain that occurred there at that time.

  • Green = less than 4 inches
  • Yellow to orange to red = 6–12 inches
  • Violet to white = 12 inches or more

Just west of Clay City, there is a large, nearly square-shaped area of floodwater in the 2008 flood image. It covers about 2,200 acres. The fact that this area became inundated indicates that it is low lying land. It’s possible that during the flood, water flowed from the nearby Eel River through a breach. The 2007 image shows this area as cropland, which is commonly low and flat. Roads, built up above ground level, could be causing the straight lines that appear around the water in the flood image.

Interestingly, this spot was once an artificial reservoir, called the Splunge Creek Reservoir, that formed in the 1800s to supply water to the Wabash-Erie Canal, which no longer exists.

In this area at near Lawrenceville, Illinois, and Vincennes, Indiana, some fields are green, some are maroon, and others are pink. Why are these fields different colors?

In this band combination, bright green is healthy vegetation, so the crops are growing well in those fields. Some fields have a faint green color; crops are growing in those fields, but they are younger than the crops in the bright green fields. The pink and maroon fields are bare soil, probably planted but there is no growth yet. The bright pink fields have slightly less moisture in the soil. In the post-flood image, some of these fields near the river that were flooded are dark pink, indicating they still contain moisture.

Floods are examples of short-term environmental change. They cause substantial damage and change for only a short time, such as a couple of weeks. The damage done to crops can last for an entire growing season, but in most cases, the landscape goes back to normal after the floodwaters recede. In some cases, however, a flood can cause more lasting change.

This flood changed the course of the Wabash River just above where it flows into the Ohio River. We have to go to a different Landsat image to see this happen, the one just south of the scene we’ve been examining. Images show a new cutoff that was formed from this flooding. About 2,200 acres of the land was rendered inaccessible by the new cutoff and shortened the river by about 7.5 miles.

Cutoffs are common on meandering rivers like the Wabash, but it’s rare to be able to witness a cutoff forming as it happens. Scientists are using this cutoff as a chance to learn more about what happens when these cutoffs develop and how cutoffs change after they form.


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