Flooding in the South Platte

Peter Stegen
November 3, 2013

In September 2013, it began to rain in Colorado. And it didn’t stop. Northwest of Fort Collins, the North Fork of the Cache La Poudre River soon carried record amounts of water. In just a few days, flows leapt from three cubic feet per second (cfs) to more than 1000 cfs when the upstream dam could not hold any more water and began to spill over.

In just a few days, many communities along the urban Front Range received record amounts of rainfall. Boulder, in particular, got almost a foot of rain in less than a day. Several other communities held similar numbers. Resulting flash floods swelled creeks and flooded roads and towns. High up in the South Platte River drainage, hundreds of mountain streams coalesced to flow out on the South Platte floodplain, wreaking havoc on downstream communities in the densely populated region.

ABOVE: A long stretch of Highway 34 in Big A long stretch of Highway 34 in Big Thompson Canyon was completely demolished by floodwaters. (Dave Showalter)

ABOVE:The South Platte River left its banks near Fort Morgan, Colo., on September 16th, 2013 after record setting rainfall fell throughout the Front Range. (Dave Showalter)The South Platte River left its banks near Fort Morgan, Colo., on September 16th, 2013 after record setting rainfall fell throughout the Front Range. (Dave Showalter)

ABOVE: Debris from the South Platte River piled onto the side of the road near Fort Morgan, Colo., on September 16th, 2013. (Dave Showalter)

ABOVE: More than 1,900 oil wells in the South Platte Basin were shut down as floodwaters rose. There were at least eleven oil spills in the South Platte River, a critical freshwater source for the Ogallala Aquifer and Great Plains agriculture. (Dave Showalter)

Colorado’s Front Range experienced record amounts of rain, causing devastating floods. Almost as fast as the rain fell, water began to move downstream towards Nebraska. Once it reached flatter ground, waters slowed down and spread out, carrying debris, waste, and chemicals through its second stage: a long, slow, muddy crawl to the eastern edge of the basin.

By late September and into October, the nearly 25,000 cubic feet per second (cfs) of water from Colorado’s Front Range slowly pushed through Nebraska: roughly 187,000 gallons of water per second.

East of North Platte, Neb., Central Nebraska Public Power and Irrigation District (CNPPID) diverts water into their supply canal, providing irrigation water for much of the state, especially south of the river. Five gates allow water to pass into the canal, each capable of moving 2,000 cfs. Sixteen gates perpendicular to the canal allow water to move down the central Platte River, delivering water to downstream water users and the Missouri River.

ABOVE: The North and South Platte rivers meet east of North Platte Nebraska, where water is diverted south to an irrigation Supply Canal. Two time-lapse cameras were set up in the area. You can see their viewshed in red.

With the benefit of advance warning, the CNPPID prepared for higher flows at the confluence of the North and South Platte rivers. They estimated about 2,000 cfs would flow into their supply canal system, while the rest would move through CNPPID gates downstream.

To help record and document this process over time, we deployed two temporary time-lapse camera systems near North Platte, Neb., one looking up the South Platte River and the other looking over the diversion dam after the confluence with the North Platte River. They both captured images every few minutes.

ABOVE: At the confluence of the North and South Platte rivers, the Central Nebraska Public Power and Irrigation District manages the flow of water. (Peter Stegen)

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ABOVE: At the confluence of the North and South Platte rivers, the Central Nebraska Public Power and Irrigation District manages the flow of water. (Peter Stegen)

ABOVE: At the confluence of the North and South Platte rivers, the Central Nebraska Public Power and Irrigation District manages the flow of water. (Peter Stegen)

ABOVE:The Central Nebraska Public Power and Irrigation District expected to move 2,000 cfs down the Supply Canal and 18,000 cfs down the Platte River. Each gate is capable of moving 5,000 cfs. (Peter Stegen)

In subsequent days, our permanent cameras along the Platte River in central Nebraska watched as water continued downstream, approaching the Missouri River.

Above the diversion dam, CNPPID continually dredges the river bottom, collecting built up sediment and sending it over the dam or collecting it in spoil piles west of the canal. The flood event tested the system, requiring quick response time to clogging debris and flushing of the system. Extra effort was made to move collected sediment over the dam while a dozer spread it out to get carried off downstream.

While the floodwaters devestated communities and created untold damage in Colorado, they had benefits downstream. After several demanding irrigation seasons the last few years, Nebraska reservoirs were able to go into the off-season at capacity much quicker than normal; water managers would say “money in the bank.” These reservoir lakes (Johnson, Jeffery, Elwood) not only provide security for next year’s irrigation season, but also satisfy recreation demands to communities along the supply canal.

In the central Platte, the scene changed dramatically. Most of the river was dry and full of vegetation from several years of drought conditions. The floodwaters helped to scour the river channel and sandbars, much like they did historically in the spring, saving conservation organizations money in mechanically clearing cottonwood seedlings, willows, etc., pushing back the yearly disking regime at least a year.

ABOVE: A tern and plover nesting site near Overton NE in the central Platte River is partially cleared of vegetation during the fall floods. Currently, mechanical means are used to accomplish this goal without traditional spring floods.

By spring, sandhill cranes will have prime roosting habitat between Kearney and Grand Island: wide open channels with bare sandbars. It remains to be seen if sandbars were cleaned enough for terns and piping plovers. The cleared sandbar’s relative elevation to typical spring flows is a key ingredient for tern and plover use.

It may be too early to tell what an autumn flood means for the river but it is certain that studies and scientific monitoring are critical for a deeper understanding. As weather and climate continue to act in variable ways, it is now more important than ever to respond and react to these anomalies. The health and sustainability of our river system depends on it.

Video and Photography by Pete Stegen

Photography by Dave Showalter

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