The Missouri River Ecosystem Exploring the Prospects for Recovery (2002) / Chapter Skim
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3. Missouri River and Floodplain Ecology
3. Missouri River and Floodplain Ecology
Pages 54-85
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From page 54... ...
This chapter describes these environmental changes in the Missouri River ecosystem, contrasting the ecological state of the river and its floodplain before and after the construction of the series of large mainstem dams and reservoirs. The physical and ecological units of today's Missouri River ecosystem are then described, followed with a review of the scientific research that forms the basis for our understanding of the ecosystem's dynamics and the consequences of human actions.
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From page 55... ...
This research and the scientific understanding of river ecosystems builds upon a longer history of research on hydrology, geomorphologic processes (the shaping of river channels by water and sediment) , vegetation dynamics, and river mechanics.
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From page 56... ...
and water volume and velocity decreased, the degraded channel would refill with deposited sediment, braided channels and meanders would become isolated from the main channel, and fresh substrates would be deposited for occupation by plants and animals. Prior to channelization and flow regulation, the lower Missouri River was braided to highly sinuous, a form naturally found in rivers with broad floodplains and heavy sediment loads.
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From page 57... ...
Based on surveys completed between 1878 and 1892, the maps in figures 3.2 through 3.4 portray the sand bars, meanders, and extensive floodplain vegetation that characterized the pre-regulation riverine ecosystem at Bismarck, North Dakota, Yankton, South Dakota, and St. Joseph, Missouri, respectively.
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From page 58... ...
of the pre-regulation Missouri River ecosystem was sustained through a regime of natural disturbances that included periodic floods and attendant sediment erosion and deposition. These disturbances, in turn, supported a variety of ecological benefits, including commercial and recreational fishing, timber, biomass fuels, wild game, trapping and fur production, clean water, medicines, soil replenishment processes, and natural recharge of groundwater.
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From page 59... ...
cesses sustained by periodic flooding in river-flooUplain ecosystems are encompassed by the contemporary concept known as "the flood pulse." The Flood Pulse Concept The concept of the "flood pulse" summarizes the effects on biota of the connections between the river channel and floodplain (funk et al., 1989; Bayley, 1995~. The flood pulse describes the predictable rising and falling of water in a natural river-flooUplain ecosystem as the principal agent controlling the adaptations of most of the biota.
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From page 60... ...
The river channel and its floodplain both depend on erosion and deposition associated with the channel's lateral migration. Inundation deposits silts and nutrients that replenish floodplain pools and backwaters.
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From page 61... ...
Periodic high flows and low flows help maintain the health of large river-floo~plain ecosystems by acting as "reset" mechanisms that reinitiate early successional vegetation and serve to limit certain faunal associations that can outcompete species normally restricted to life within a channel (Cummins et al., 1984~. Fish spawning, insect emergence, and seed dispersal are commonly triggered by
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From page 62... ...
, where tributary influences partially restore pre-regulation flows to the river. Below Fort Peck Dam, for example, the median high flow was cut in half following the dam's closure (Shields et al., 2000~.
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From page 63... ...
75RRRR 65RRRR BRRRRR 55RRRR 5RRRRD 45RRRR 4RRRRB 35RRRR BRRRRD 25RRRR PRRRRB 15RRRB 1RRRRD 5RRRD R ~ R ~TTTTTTT~ ~ r T I [ [ 1 1 T T T [ T T [ [ 1 1 1 T T T [ T [ [ [ 1 1 1 T T 1 [ T [ [ [ 1 1 T T T [ r T [ [ r l l T T T [ T T [ [ 1 1 1929 1939 1949 1959 1969 1979 1989 FIGURE 3.6 Missouri River discharge at Hermann, Missouri. SOURCE: USACE, undated.
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From page 64... ...
. From Ponca State Park downstream to the Big Sioux River, the river channel is stabilized.
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From page 65... ...
From 1940-1952 (the period from the closure of Fort Peck Dam until the closure of Gavins Point Dam) , the average annual sediment load transported past Omaha was 148,930,000 metric tons.
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From page 66... ...
of the river channel. This lowering of the water table effectively drains water from oxbow lakes and wetlands.
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From page 67... ...
Lateral migration of river channels can occur in areas below dams: however. meandering rates have been markedly , .
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From page 68... ...
These changes, combined with other human activities in floodplain areas, have produced an ecologically impoverished ecosystem. MISSOURI RIVER ECOSYSTEM PHYSICAL AND ECOLOGICAL UNITS Scientific investigations today are conducted in an ecosystem that changed greatly during the twentieth century and that today is fragmented into distinct physical and ecological units.
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From page 69... ...
Some examples are instructive. Missouri River Tributaries Yellowstone River The Yellowstone River flows 675 miles through Montana to its confluence with the Missouri River at the North Dakota border (Figure 3.10~.
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From page 70... ...
The Bad River is small and intermittent and therefore provides only limited ecological benefits to the Missouri River mainstem when compared to the flows of the Yellowstone River. Moreover, there is only a short distance between the confluence of the Bad and Missouri rivers and Lake Sharpe, leaving little of the sediment-deprived Missouri River downstream of Oahe Dam to benefit from the Bad River's input of sediment-laden water.
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From page 71... ...
This reach without storage reservoirs extends from the Missouri River source near Three Forks, Montana, downstream to Canyon Ferry Reservoir, a distance of about 30 miles. However, the much longer reach from Canyon Ferry Dam to Fort Peck Lake is only mildly regulated because of the comparatively small storage capacity of Canyon Ferry Reservoir relative to total river flow and the long distance between Canyon Ferry Dam and the next downstream reservoir (Fort Peck)
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From page 72... ...
In some cases, the headwaters of the mainstem reservoirs extend nearly to the tailwaters of the next upstream dam; there are few remnant floodplains from Lake Oahe downstream to Fort Randall Dam. In other cases, reservoirs are separated by large stretches of river (e.g., section 3, from Fort Peck Dam downstream to Williston, North Dakota; Figure 3.10~.
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From page 73... ...
; Pierre, South Dakota (upstream of Lake Sharpe) ; and Bismarck, North Dakota (upstream of Oahe Reservoir)
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From page 74... ...
The river's upper portion in South Dakota, Nebraska and Iowa has degraded because of erosive water releases from upstream dams, the trapping of sediments in mainstem reservoirs, and insufficient flows to accomplish lateral channel readjustment. By contrast, the Missouri's lower reaches (especially downstream of the Platte)
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From page 75... ...
Engineering works on the river's main channel have resulted in significant ecological changes in the channelized reaches. Construction of revetments has greatly narrowed and deepened the channel and has fixed its location.
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From page 76... ...
The shoveinose and the pallid sturgeon are among the native species that have nearly disappeared in the reservoirs. The paddlefish has also been extirpated from much of the reservoir system, with remnant populations above Fort Peck Lake, at the confluence of the Yellowstone and Missouri rivers, and near the mouth of the Niobrara River.
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From page 77... ...
Pallid sturgeons were thought to live primarily in large, turbid rivers such as the Missouri, and the Mississippi River downstream from its confluence with the Missouri. It utilized overflow areas on the floodplain, backwaters, chutes, sloughs, islands, sandbars, and main channel banklines, pools, and snags (Dryer and San~vol, 1993~.
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From page 78... ...
They have since declined by as much as 98 percent in some locations in the river (Figures 3.11 and 3.12; from Hesse, 1994~. Sauger were important sport fish of exceptional food quality, and recreational anglers fished for sauger before the mainstem dams were built.
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From page 79... ...
One of the few comprehensive surveys of the floodplain forest flora found 220 species of vascular plants growing in the remnant river section between Garrison Dam and Oahe Reservoir in North Dakota (Keammerer et al., 1975~. This inventory was conducted long after extensive forest clearing had occurred and did not include a comparably rich flora of wetland plants found in non-forest communities on the floodplain.
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From page 80... ...
No equally comprehensive inventories of floodplain vegetation have been published in downstream sections of the river; however, general descriptions are available and general ecological relationships are known. Downstream from North Dakota, the overall floodplain flora becomes considerably richer, particularly the woody component.
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From page 81... ...
More recently, due partly to the declining value of wild fur, beaver are more numerous even along portions of the channelized reaches (Larry Hesse, River Ecosystems, Inc., personal communication, 2001~. Botanical research from 1960 to 2000 on the upper Missouri River centered on the effect of dams on floodplain forest succession, particularly
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From page 82... ...
, and the remnant floodplain between Garrison Dam and the headwaters of Oahe reservoir, near Bismarck, North Dakota (Johnson et al., 1976~. Later investigations of cottonwood regeneration in the reach of the Missouri River's section designated as a national monument in Montana (formerly designated Wild and Scenic)
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From page 83... ...
The cottonwood forests that remain on the floodplain between and immediately below dams cannot be sustained by the current low river meandering rates. Both erosion and deposition rates (an index of river meandering rate)
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From page 84... ...
Reily and Johnson (1982) found that seasonal shifts in flow, the near-elimination of overbank flooding, and a lowering of the water table below the floodplain in late spring, reduced the growth of trees occupying remnant floodplains between reservoirs.
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From page 85... ...
The hydrologic connections between the river channel, floodplain, and backwater areas were greatly disrupted. In the channelized portions, river meandering was eliminated and the ecological diversity of the river and floodplain was greatly simplified.
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