23 October 2010

Early Autumn Survey of Water Conditions at Mitigation Sites

A survey on September 27th indicated the extent of wet lands along the Missouri River between Langdon Bend and northward to near Sioux City in Nebraska.

With ongoing high releases of water from Gavins Point Dam, the plane survey was done to evaluate water conditions at the Missouri River Recovery Program project sites, according to Lynn Heng, a Natural Resource Specialist in the Omaha District of the Army Corps of Engineers.

Overall, "it was wet in the southern extent of the survey area, and extremely wet in the northward region," Heng said of the six-hour aerial survey.

The flight allowed Heng to become familiar with the sites and to get a perspective of their conditions. He is responsible for management of several of the land tracts. Because of the higher-than-normal water levels, he also wanted to get a preliminary assessment of any damage, such as excessive sediment deposition in backwaters and chutes, and to see how much timber debris may have accumulated.

At some of the prominent chutes, many which have been created in recent years in conjunction with the Corps' Missouri River Recovery Program, the high water was actually having a beneficial effect.

The chutes were built with a particular width and bottom slope, and the natural erosive influence of the river waters has altered these features to where they mimic what would have been present when the river was unaltered in the centuries prior to the 1940s.

Heng called this adaptive management, where current engineering design and methods are used with projects.

Variable Conditions

Conditions at the different project sites varied during the survey, with Heng noting several prominent findings:

- Snyder Bend: "really inundated"
- Glovers Point Bend: the majority of the area, which now includes a dredged backwater and chute, was completely inundated. "The bank of the river was not even visible in many locations," Heng noted.
- Tieville-Blackbird-Upper Decatur Bends: The rock-armored outflows were working as planned to allow the high levels of water to flow outward from the project sites, and back into the river. "It was neat to see the system functioning as it was meant to be," Heng said. With the high level of water, there has been no need to operate electric pumps which are typically used to augment autumn water conditions at these sites.
- Bullard Bend: extremely wet with a "good amount" of standing water over much of the area. This is one of the newest Corps' project sites and includes a backwater.
- Sandy Point Bend: this site would normally have an isolated, off-channel lake, but with the greater than average flows, the bit of a lake looked as if it was part of a continual chute adjacent to the primary channel.
- South of the Platte River confluence, at Tobacco Island, the lowland next to the river was wet, but not inundated, with the chute still obvious.
- At Van Horn's Bend, one of the most recent properties acquired by the Corps, what had been cropland was flooded. A big help in creating this situation, Heng said, was a notch placed in a riverside levee which allowed the water to flow onto the land. This 240-acre tract was still really wet in early October.
At this particular site, once the land dries, the plans to establish some native grasses and limited tree plantings, and where cottonwood saplings have sprouted, allow them to continue to thrive. A chute may also be created here, Heng noted.
- Hamburg Bend: the lowland was dry in April, allowing vehicular access to the river, but in July it was inundated with water from the levee to the river, and this latter condition continued into early autumn.

As water levels recede, Heng will assess the conditions at the different sites and determine what maintenance work may be necessary. Typical work has had to be postponed during to the high-water, and other work such as placing rip-rap has also been delayed.

Overall, the aerial reconnaissance trip was very beneficial in providing a snapshot of current water levels and an idea on what the Corps can expect to see once the high flows subside.

Water releases from Gavins Point Dam are expected to continue at about 45,000 cubic feet per second, into December.