Sediment in the Henry?s Fork

By STEVE TRAFTON
Henry’s Fork Foundation Director

When anglers and other observers of “the Ranch” — the Henry’s Fork as it flows through Harriman State Park — discuss the differences between the river of the past and today’s river, the subject of sediment almost always comes up. Again and again, visitors with a long perspective on the river and its fishery will mention that places that formerly had deep undercut banks, or midstream channels, or abundant aquatic vegetation, or pools, are today too shallow to provide good trout habitat, and devoid of trout. Sediment, specifically sediment deposited after the 1992 drawdown of Island Park Reservoir, is to blame — or so many anglers feel.

While that may well be true up to a point, history suggests that the full story is far more complicated.

Island Park Reservoir acts as a sediment trap; all the material washed downstream from the Henry’s Fork’s northernmost headwaters, in particular from the Henry’s Lake outlet and the tributaries entering the river upstream of Mack’s Inn, settles to the bottom of the reservoir as the river’s current slows, loses energy, and can no longer move it. Nobody knows exactly how much sediment has accumulated in the reservoir since Island Park Dam was completed in 1939, but we do know that under certain conditions accumulated sediment can regain momentum and move downstream, especially during reservoir drawdowns. In 1992, the reservoir was reduced to a pool of 270 acre feet (full capacity is 135,000 acre feet), and an estimated 50,000 to 100,000 tons of sediment were released into the Caldera section of the Henry’s Fork. This event received widespread public attention, and research, photos, and anecdotal evidence confirm that huge volumes of material ended up in the Ranch.

But was that the moment that everything changed, the equivalent of history’s transition from B.C. to A.D., the point after which the Ranch fishery, and all of the aquatic variables on which it depends, began to decline? In fact, Island Park Reservoir has been drawn down under 10,000 acre feet on two other occasions, in 1979 (430 acre feet) and 1966 (5,500 acre feet). Although these events received less attention than the 1992 release, we must assume that they, too, were attended by the movement of large volumes of sediment out of the reservoir into the Box Canyon and through to the Ranch. Yet, both anecdotal and scientific data tell us that the aquatic vegetation, insect hatches, and fish populations were thriving, for several years at least, in the aftermath of the 1979 release. My point here is not to diminish the impact of the 1992 sediment event, but rather to point out that it cannot be considered in isolation.

Water managers try to avoid reducing the reservoir to levels below 10,000 acre feet, but angler reports of “off-color” water suggest that some material does come through the dam at higher reservoir levels than 10,000 feet. Whether this sediment is organic (and therefore subject to decomposition) or inorganic, and at what reservoir levels this mobilization occurs, are not fully understood. Nor do we fully understand how sediment is transported through the Ranch. This was evaluated in 1994 and again in 1997, and flushing flows were made over a four day period in 1995. The sediment transport studies indicate that the river moves sediment well under favorable (high) flow conditions, but that typical (normal) flow scenarios may result in a net gain of sediment. The flushing flows of 1995 probably moved considerable amounts of sediment, but how much and how far are unknown, and it is likely that much of the material was easily-mobilized sediment remaining from the 1992 reservoir drawdown. These studies unfortunately lack the larger context necessary to definitively increase our understanding of how sediment is transported in the Caldera.

Why does sediment matter? In fisheries terms, sediment concerns generally fall into either of two categories: loss of spawning habitat, and loss of pool and/or overwintering habitat. Some spawning gravels in the Caldera section of the Henry’s Fork may have been filled in with fine sediment in 1992 and other years; however, research shows that so many juvenile trout are produced in both the Box Canyon and at Last Chance that there has been no effect on the overall trout population.

Spawning success is not a limiting factor in the Caldera section. Sediment deposition can also alter both the pool habitat favored by trout of all ages and the areas along the stream’s margins vital to juvenile trout survival. The extent to which this has occurred in the Caldera, and its effect on the trout population, are difficult to gauge in the absence of baseline data, and because other factors, in particular changes in aquatic vegetation types and densities, may play an equal or greater role.

It is important to note that sediment transport is a naturally-occurring and ecologically important phenomenon in streams, and that sediment deposition should not be assumed to be a negative event. Indeed, stream reaches below dams can become “sediment-starved” when the natural process of sediment transport into the reach is cut off artificially, with the potential for a variety of negative results. That said, on the other side of the spectrum, as soon as sediment inputs to a stream are increased due to manmade causes such as grazing or poorly built infrastructure — or the accumulation and subsequent sudden release of sediment stored behind a dam — the natural balance is upset.

That is the primary reason that the Henry’s Fork Foundation has spent considerable time and effort designing, implementing, and now monitoring a stream channel restoration project on the Henry’s Lake outlet as it flows through the Flat Ranch (owned and managed by The Nature Conservancy). The outlet is a major source of sediment to the upper Henry’s Fork and Island Park Reservoir, much of that sediment due to artificial channel straightening undertaken decades ago. Stream restoration projects like the Foundation’s are the first step towards reducing the amount of excess sediment being deposited from the project site all the way down to and into Island Park Reservoir.

This is part of the online edition of Henry's Fork Country.

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