UNL Climatologist: Nebraska Impacts from Colorado Flooding

UNL Climatologist: Nebraska Impacts from Colorado Flooding

September 19, 2013

This week the nation has been closely following the dramatic impacts of the Colorado flood that resulted from 12-18 inches of rain along the front range of the Rocky Mountains. Colorado State Climatologist Nolan Doesken indicated that numerous stations around Boulder recorded a year’s worth of moisture during this 1000-year event. In a span of three days, Boulder went from the driest year to date to the wettest year to date on record.

The flood waters along the front range of the Rockies have begun to recede, but South Platte communities downstream continue to closely monitor rising river levels as the water moves toward the Missouri River. Major flooding is expected in Nebraska from the Colorado border to North Platte through this weekend. By early next week, there is the potential to see flood conditions from North Platte to Columbus and mid-week for the Ashland to Plattsmouth area.

Official flood forecasts are somewhat uncertain due to river gauges being destroyed from Denver to Fort Morgan. In addition, the river gauge at Roscoe is not currently operational due to cost-saving measures undertaken by the U.S. Geological Survey in response to the federal budget sequestration earlier this year.

Compounding the loss of river gauges is that the South Platte has not experienced a flood event approaching this magnitude in at least 30 years and vegetative growth within the river channel will likely exaggerate the surge of flood water. Not only will debris from front range communities be moving downstream, but the flood waters are expected to scour out the primary river channel and add additional debris to the flood surge.

Fortunately, the river gauge northeast of Fort Morgan is operational and indicates that the South Platte River flow peaked on Monday at 28,000 cubic feet per second. The river flow was a hundred-and-eight-fold increase of normal, but had dropped to 22,000 cfs by Wednesday. Because river flows have been virtually non-existent the past two summers due to extreme drought conditions, the river channel aquifer will be able to absorb a portion of this flood surge.

Thankfully, most of the flood impacts in Nebraska are likely to occur across agricultural areas and are not expected to significantly impact metropolitan communities in close proximity to the Platte River. Once flood waters recede, it is likely that producers in flood prone areas will have to address widespread debris cleanup of pastures and crop fields.

Looking Forward

Among all of the devastation that has and will likely occur with this historical flood event, there are some potential benefits that could materialize after the water recedes. The South Platte channel will be scoured of vegetation and a greater volume of water will flow through the channel instead of being tapped by growing vegetation. Colorado reservoirs within the South Platte Basin are now full, so water supply issues for metropolitan areas have been eliminated.

In a typical year, Colorado reservoirs will experience a continual drawdown from their spring peak through the late winter months before snowmelt begins. Because South Platte basin reservoirs are now full, water managers will need to make room for the 2014 snowmelt season through water releases that eventually will make their way downstream to Nebraska. With soil surfaces currently saturated in mountainous areas, a greater than normal portion of this winter’s snowpack will run off and subsequently increase flows along the South Platte River next spring.

The expected increase in flows this spring also may impact Lake McConaughy water levels even though none of this flood water will directly flow into the reservoir. The increase in flow along the South Platte this spring could give the Central Nebraska Public Power and Irrigation District (CNPPID) the flexibility to divert water into irrigation canals instead of releasing water from McConaughy. Thus, CNPPID may be in a similar position to 2011 when above normal South Platte flows allowed them to capture spring snowmelt on the north branch of the Platte River that would enter into Lake McConaughey. Typically, a portion of the North Platte flow into Lake McConaughey would be diverted and passed downstream to fill early season irrigation obligations.

Al Dutcher
Extension State Climatologist