America’s Most Endangered Rivers for 2013: Niobrara River
Nebraska, South Dakota, Wyoming
At Risk: Property, crops, and public safety
Threat: Sediment build-up and flooding
The Niobrara River is an oasis for paddlers, anglers, and wildlife. A major tributary of the Missouri River, the lower portion of the Niobrara is protected as a federal Wild and Scenic River. The Lower Niobrara is increasingly threatened by too much sediment backing up in the upper reaches of Lewis and Clark Lake behind the Missouri River’s Gavins Point Dam.
The sediment is raising the level of the Niobrara and threatening local communities with flooding. To safeguard the Wild and Scenic Niobrara and its communities, the U.S. Army Corps of Engineers must improve sediment management within the Missouri River system and must prioritize funding for this critical issue in their Fiscal Year 2015 budget.
Dams and other flood control structures built in the mid-1900′s on the Missouri River have caused sediment to accumulate in the reservoir system. When sediment builds up on the riverbed it raises river levels, creating flooding issues for tributaries like the Niobrara River. At one time, most of that sediment was flushed out of the system to the Mississippi River where it helped build coastal wetlands along the Louisiana Delta— keeping saltwater intrusion at bay. Now, reservoirs behind dams act as sediment traps, slowing the flow of the river and allowing sediment to settle, accumulate, and consequently deplete reservoir flood storage capacity. At the same time, water management for navigation using water released from dams has caused lowering of the riverbed below Gavins Point Dam. Without sediment replenishment, it takes more water to serve downstream authorized purposes.
The sediment build-up is so extreme at the confluence of the Niobrara and Missouri Rivers that the overall level of the local water table has increased substantially. This leads to flooded cropland and basements, and greatly impacts boating and other recreation. Flooding due to sedimentation forced relocation of the Village of Niobrara in 1973. Downstream from the confluence, Lewis and Clark Lake is expected to lose 50 percent of its water storage capacity by 2045 due to sediment accumulation in the reservoir— to date, it has already lost 30 percent of capacity.
As the sediment builds within the system, the Lower Niobrara is slowly losing the seeps, springs, riparian forests, prairies, and canyons that characterize this Wild and Scenic River. Only a few of the great cottonwood trees in the confluence area have survived the recent sustained high waters. The USACE has purchased thousands of acres of riverside land to limit future liability, but this is not the long-term answer. The underlying sediment problem must be addressed now to secure a future for this Midwestern treasure.
What Must Be Done
Conflicts among authorized uses of the Missouri River have made reaching a solution to this problem a challenge. The USACE has released several studies outlining various alternatives for managing the increasing sedimentation on the lower Niobrara. These studies use regulated flows of the Missouri River to flush the accumulating sediment below Gavins Point Dam.
Now, the USACE must develop and implement a sediment management plan for the area of the confluence of the Niobrara and Missouri rivers. This plan must implement the minimum flow and the shortest duration needed to transport sediment while minimizing downstream flooding risks. The plan should also include appropriate modifications of Gavins Point Dam and methods to suspend existing sediment deposits.
Furthermore, the USACE must dedicate funding in its Fiscal Year 2015 budget to implement this plan. Without such a plan in place, this one-of-a-kind river will be buried, and its Wild and Scenic qualities will be gone forever. There is increasing urgency to preserve the economic, recreational, natural, and historical assets of the Missouri River Basin for future generations.