Will Floods Cause Loss of Lake Oahe Smelt Population ?

Last week we walked through the challenges that wildlife face in responding to floods.

Fish face challenges too, though obviously not of the same nature as land-dwelling animals. Here’s a selection of some of the more common questions the North Dakota Game and Fish Department has answered over the past several months regarding the Missouri River System.

What will be the long-term impacts of Floods to walleye and smelt?

If history is an indication of what to expect, Game and Fish is concerned about the potential loss of smelt through Garrison Dam and especially Oahe Dam late this summer.

Our most recent experiences with high flows, albeit much lower than current flows, was in the 1990s. On Lake Sakakawea, the water intakes for Garrison Dam are located in deep waters, where smelt and walleye don’t reside for long periods of time. Despite what appeared downstream (Garrison Tailrace) as large numbers of smelt being entrained from Lake Sakakawea in 1997, that loss really didn’t see a negative impact on the population as a whole.

Lake Oahe was a different deal. The water intakes for Oahe Dam are located mid-way down the water column. In 1997, as the thermocline set up at the same level as the intakes, large numbers of smelt passed through the dam due to their affinity to the thermocline habitat. South Dakota biologists estimated that the vast majority of Lake Oahe smelt went through Oahe Dam that summer.

Without that smelt forage base, the walleye population suffered for a number of subsequent years, resulting in slower growth, poor body condition, and overall higher mortality rates.

Lake Oahe currently has a high abundance of younger walleyes (less than 14 inches) due to good natural reproduction in recent years. If we lose the Oahe smelt population again, which was only recently reaching a satisfactory level of recovering from the last flush, those younger walleyes will lack the forage they need to grow to a desirable size for anglers in the years to come.

In the short term, this could lead to good fishing, as the walleyes will be hungry. But in the long term we will begin to see skinny fish that are smaller than average – and we will lose far more fish to natural mortality (starvation) than to angling.

Our department, along with the South Dakota Department of Game, Fish and Parks, will continue to monitor smelt and walleye populations on both reservoirs.

Will the reservoir and river habitat change?

Absolutely. Lake Sakakawea and the Missouri River above it will likely change the least, as they annually experience a true “June rise” from the uncontrolled Yellowstone River.

Lake Oahe, especially the upper end from immediately south of Bismarck to approximately the Hazelton/Ft. Rice area will experience more sediment deposition than ever. It is possible that many of the distinctive backwater areas and oxbow lakes will be filled with sediment after water levels recede. Both lakes Sakakawea and Oahe will also experience record amounts of high-bank erosion.

The 70 or so miles of the Missouri River between Garrison Dam and Lake Oahe will experience the most profound changes. High releases over a prolonged period will greatly scour the river bed. Many islands and sandbars that populated the river for the past half century will be displaced, and in some cases, the main river channel may reclaim some of its former self by incising a new path of least resistance.

Sandbars, islands and drop-offs are key fish structure and in large part responsible for the world-class recreational fishery that the MRS provides. They will still exists, but their location and size will change.

Doug Leier is a biologist with the Game and Fish Department. He can be reached by email: dleier@nd.gov.

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