The Missouri River Story
GEOGRAPHY
The Missouri River drains one-sixth of the United States and encompasses 529,350 square miles. It flows 2,341 miles from its headwaters at the confluence of the Gallatin, Madison, and Jefferson Rivers in the Rocky Mountains at Three Forks, Montana, to its confluence with the Mississippi River at St. Louis, Missouri.
The basin is home to about 10 million people from 28 Native American tribes, 10 states (Colorado, Iowa, Kansas, Minnesota, Missouri, Montana, Nebraska, North Dakota, South Dakota, Wyoming), and a small part of Canada.
Precipitation in the basin varies from an annual mean of 40 inches in the interior highlands of the Missouri Ozarks to 10 inches in the dry upland plains of North and South Dakota, Wyoming, and Montana. The basin’s elevation drops from 14,000 foot peaks at its northwestern boundary to about 400 feet where it joins the Mississippi.
HISTORY
The pre-development Missouri River represented one of North America’s most diverse ecosystems with abundant braided channels, riparian lands, chutes, sloughs, islands, sandbars, and backwater areas. These riverine and floodplain habitats were created and maintained by erosion and deposition which continuously reshaped the channel and floodplain.The Missouri carried high sediment loads, earning it the nickname "Big Muddy." Two programs, the Pick/Sloan Plan (1944) and the Missouri River Bank Stabilization and Navigation Project (1945) transformed the free-flowing river into a system of main stem reservoirs in the upper river and highly altered riverine reaches influenced by self-channelization, bank stabilization, and regulated flows in the lower river. Today, 35-percent of the Missouri River is impounded, 32-percent has been channelized, and 33-percent is unchannelized.
In addition to the main stem modifications, the river is influenced by construction of levees along the lower river and major tributaries, channelization of floodplain tributaries, and an extensive reservoir system in the large tributary basins of the Platte, Kansas, and Osage Rivers.
These changes have significantly altered the Missouri River ecosystem. In the upper river, a new ecosystem has been created with the deep water reservoirs replacing the free-flowing river and inter-reservoir reaches affected by lower water temperatures and reduced sediment loads. In the lower river, channelization has eliminated sandbars, depth diversity, and river connections with off-channel side channels and backwaters. The historical flow regime has been transformed with spring high flows now captured in reservoirs and low summer and fall flows augmented with reservoir releases.
All of these changes have lowered populations for many river fish and bird species, some to the extent that they are federal or state-listed as endangered, threatened, or species of special concern.
RESERVOIRS
The Missouri River reservoir system is the largest in the United States with a storage capacity of 74 million acre feet and a surface area exceeding one million acres. The six dams built in Montana, Nebraska, North Dakota and South Dakota transformed one-third of the Missouri River ecosystem into lake environments.
The original development plan called for a series of reservoirs to be built in order to lessen the effects of flooding in the lower basin and provide flows for navigation below Sioux City, Iowa. Upper basin benefits included irrigation and power generation. Though irrigation never developed as planned, economically important sport fisheries in the reservoirs and below the dams have developed.
Great quantities of sediment and organic materials flow into the reservoirs and are trapped behind the dams, reducing reservoir storage capacity and sediment transport below the dams. Dams block native fish migration to spawning grounds and modify the flow regime in the river system.
Deltas are formed at the reservoir headwaters from sediment mobilized in the inter-reservoir reaches and arriving from upstream tributaries. Deltas reduce reservoir storage and channel carrying capacity. Extensive wetlands have developed in the reservoir headwaters, providing excellent waterfowl and waterbird habitat and spawning areas
for fishes.
INTER-RESERVOIR REACHES
Inter-reservoir reaches run from directly below the dams to the headwaters of the next downstream reservoir. While these reaches maintain some of their pre-development channel morphology, they are affected by altered water temperatures, unnatural water level fluctuations, and changes in sediment and nutrient transport.
Sediment "hungry" water released from the reservoirs degrades or cuts the river bed below the dams lowering groundwater tables and dewatering side channels, sloughs, and backwaters connected to the channel. Deep reservoir releases lower water temperatures in reaches below the dams. Both of these factors interfere with native fish spawning and development.
Water levels in inter-reservoir reaches can fluctuate dramatically because of hydropower and flood control operations. Human encroachment in the floodplain of these reaches is creating a demand for additional flood control and bank stabilization.
FLOW REGIME
In the typical pre-development Missouri River flow regime, a flood pulse resulted from rain and melting snow runoff, first in March from the Great Plains and then during late June from the Rocky Mountains. Flows declined through the summer and fall reaching their low point in late December.
Native fish and wildlife evolved with this historical flow regime and depend on it to meet their different seasonal habitat and reproductive needs. Today a spring flood pulse is suppressed via reservoir storage, while dam releases provide higher river flows from July through November, eliminating summer/fall low-water flows.
Seasonally inundated backwaters and wetlands historically provided food and habitat for native river fishes. The suppression of high spring flows has prevented recharging of these areas, reduced nutrient cycling and transport, and accessibility to floodplain and nursery habitats for fishes.
In relation to pre-development conditions, few high elevation sandbars form because of the suppression of high flows which are necessary to create them. Sandbars that do remain become covered with unwanted vegetation because the scouring flows needed to clear them are unavailable. Native fish spawning cues once triggered by increasing water temperatures coupled with rising river stages have been lost within many river reaches.
CHANNELIZATION
Historically, the "Big Muddy" changed course. The channel relocated over 2,000 feet or more a year in some places and deposited huge amounts of silt in other places. It is estimated that 11 billion cubic feet of sediment were carried past St. Charles, Missouri in 1879 — enough to cover a square mile of ground 200 feet deep.Banks along the river would erode 200 to 300 feet during a single rise of the river. It was the movement of this sediment that created braided channels in the meandering river, hampering navigation and the permanency of bottomland farms and river towns.
The Rivers and Harbors Acts of 1912, 1917, 1925, 1927, 1930, 1935, and 1945 each affirmed the desire of the floodplain occupants, the basin’s elected officials, and the federal government to tame the river for navigation, development, and flood control.
The Missouri River Bank Stabilization and Navigation Project created one stabilized channel from the numerous small channels. The plan entailed concentrating the water flow and shaping it in smooth easy bends so that the energy of the flowing water scoured out a deeper, more efficient, navigation channel. Officially completed in 1981, 735 miles of the Missouri River from Sioux City, Iowa, to St. Louis, Missouri have been channelized or stabilized by the plan, allowing urban and agricultural development of the floodplain.
From bluff to bluff, the river-floodplain below Sioux City, Iowa, covers 1.9 million acres. Historically, the river meandered across more than one-fourth of this floodplain acreage. This "meander belt" contained a variety of fish and wildlife habitats including wetlands, sandbars, wet prairies, and bottomland forests. Seasonal floods provided the water needed to replenish shallow-water habitats used for fish and wildlife breeding and growth.
Channelization shortened the river 72 miles, resulting in a loss of 127 miles of river shoreline habitat. Aquatic habitat was lost as 168,000 acres of sediment accreted behind the wing dikes, forming new land. Nearly 354,000 acres of meander belt habitat were lost to urban and agricultural floodplain development. Levees, built to protect against flooding, allowed floodplain property investments. Levees isolated riverine off-channel habitats and wetlands from the river.
The damage to fish and wildlife habitat was acknowledged in 1986 when the Corps was authorized to implement the Missouri River Fish and Wildlife Mitigation Project. The goal of the project is to acquire and restore 28,000 acres in Iowa, Kansas, Missouri, and Nebraska. This equals approximately five-percent of the habitat lost as a result of the Missouri River Bank Stabilization and Navigation Project.
This Missouri River Story is from the Missouri River Environmental Assessment Program document published in June, 1998. Click here to view the original document.
