Thursday, September 19, 2013

All That Water Has to Go Somewhere

The flooding in Colorado caused by last week's unprecedented rainfall is subsiding. While the worst of the actual flooding has passed through Colorado, the tremendous amount of damage caused will take months to years to repair. Some of the landscape has been re-shaped and may remain that way. An unimaginable amount of water flushed through northeastern Colorado. Where is it going?

Eventually, some of that water will end up in the Gulf of Mexico. The flood surge is currently moving down the South Platte River in Nebraska, eventually to the Platte River, then the Missouri, and then into the Mississippi.

The drainage basin of the South Platte River
The South Platte River is located in Colorado and Nebraska. Its drainage basin includes much of the eastern flank of the Rocky Mountains in Colorado, including the Front Range and the eastern plains. The source of the South Platte is just south of Denver. It also drains a portion of southeastern Wyoming in the vicinity of the city of Cheyenne. It joins the North Platte River in western Nebraska to form the Platte, which then flows across Nebraska to the Missouri.

The rain last week fell not only over the tributaries of the South Platte such as the Big Thompson and Poudre Rivers (and many others), but very near its source.

The South Platte drainage basin with the approximate areas of the heavy rain.
The darker green areas are those that received the largest amounts of rain.

The progress of the floodwaters from the rain in Colorado can be tracked by the river gauges along the South Platte. This flooding has not been the often gradual rise in river levels you might typically see along the larger rivers in the spring. This flood is more of a pulse, a rapid rise in a short amount of time. This hydrograph from Roscoe, NE, just east of Ogallala, shows the rapid rise in river level. The river rose 4 feet (from 1.6 feet to 5.6 feet) in just two hours, and 6 feet in less than 12 hours.  It's expected to rise another 5 feet cresting at a record 12.5 feet today.

Hydrograph from Roscoe, NE.
Source: NWS Advanced Hydrologic Prediction Service

Similar rises have and will occur along the river as the surge of floodwaters moves down river. However, as the water moves into the Platte River and eventually the Missouri the rises will be more gradual.  Unfortunately, the river levels will fall a lot more slowly than they came up due to the huge volume of water draining out of Colorado.

Here is a satellite image of eastern Colorado and western Nebraska where you can actually see the leading edge of the floodwaters moving down the river. The high contrast between the swollen parts of the river and downstream is due to the large amount of sediment and debris carried by the floodwaters and the wider channel.

Source:  NWS North Platte, NE

The image below was compiled by the NOAA Cooperative Institute for Meteorlogical Satellite Studies (CIMSS) at the University of Wisconsin-Madison. This image is a loop of the unannotated image above with a second image taken on September 18.  You can see the leading edge of the flood has crossed into Nebraska on the second image.

To see the full-size image of this loop (large file), go to the CIMSS Satellite Blog.

You can follow the progress of the floodwaters through the river system on the NWS Advanced Hydrologic Prediction Service web site.

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