Community July 29, 2008
Rebirth of a River
The Big Wood River Better the Second Time Around

Anglers from around the county, as well as locals, fish the Big Wood River, making it one of the economic drivers that create and sustain the tourist economy in the Valley. But it is also a dearly loved neighbor to the communities that reside along its banks. Families enjoy hot, summer days swimming the Big Wood’s waters; cyclists travel the bike path that winds along its banks, and many a local has nurtured a passion for stalking the trout that dart between its pools and riffles.

In May of 2006, the Big Wood River unleashed its highest ever recorded volume, peaking at 7,500 cubic feet per second (cfs) on May 21. An unusually heavy snowpack, warm temperatures, and precipitation during the week before the flood contributed to the water’s historic rise. The river remained above flood stage for just under a week, transforming the river channel with swiftly coursing high water. What local communities learn from this flood and how they respond to the river in the coming years can positively influence the river and the communities it sustains. The powerful flood of 2006 shows, above all, that living with the river and learning to accommodate its changes will improve our local communities and the health of the river. And as a major factor in the purity and amount of the water in our aquifer, the river’s health directly relates to the public health of our local communities. 

High flows leave a healthier river behind.

Like a child entering adolescence, the river expressed itself in surprising ways this spring. Local floodplain residents experienced flooding in their yards, basements and neighborhoods; cities also experienced flooding of parks, streets and other infrastructure. This spring’s convergence of weather, snowfall and timing allowed flooding to reach a level that Jim Bartolino, a hydrologist with the U.S. Geological Survey calculates as “having a 0.5% chance of occurring in any given year,” according to USGS data for the period of record since 1915. The high water left behind a new and healthier river system as well as a unique opportunity to build on this newly invigorated river by shaping future policies. Previous restoration efforts on the Big Wood leave a few clues as to how local communities can learn to live with this new river and better prepare for future flooding. Learning to live with the new river—allowing it to change and evolve in its floodplain—will mean a healthier river and possibly an easier high water season for floodplain property owners in the future.

What makes a river healthy?

Rivers rebuild themselves each year as high mountain snows melt and course through the system to create new habitat for fish and bring other benefits to the river and its floodplain. The floodplain is the relatively flat land along the river that is often filled by water during high water events. Spring runoff builds new areas along the river, encouraging cottonwood seeds to take root; scours old algae off the riverbed to allow new populations of insects to thrive; fells cottonwood trees that become in-stream wood; picks up debris; and revives old side channels that are critical for trout spawning and rearing. Additions of in-stream wood and the movement of the river form new pools for fish. Shallow, slow-moving water in the floodplain (known as sheet flooding) is filtered by plants and recharges the aquifer for local drinking water supplies. Sheet flooding allows dirt and fine sediment in the stream to settle in the floodplain and the riparian area bordering the river, which enhances the growth of the native plants. Flooding and the changes associated with high water are part of a natural process that keeps the entire river system more healthy and productive. >>>



History of the Big Wood and human impact on the river.

The Big Wood is a fantastic river to fish in part because it is free-flowing for approximately 62 miles until it reaches Magic Reservoir south of Bellevue. Dams do not exist on the river above Magic Reservoir. Yet, its status as a wild section of river has encouraged efforts to control it around existing infrastructure and developments. However, the river’s ability to move within the floodplain is key to its health. Healthy rivers move in a sinuous S-shaped pattern, meandering back and forth across the floodplain to achieve an energetic equilibrium. Past efforts at flood control have resulted in diking along the river banks, filling in the floodplain, and installing riprap (large, angular rock along the banks), to limit the river’s ability to meander in the floodplain. Over time, these actions have straightened out the river channel, stretching the S-shaped curve the river follows over a greater distance. This straightening is known as “channelization.” Channelization permanently restricts the river to one channel, eliminates the natural meandering of the river, and effectively cuts the river off from its floodplain.

This year’s flooding has countered some of the effects of past efforts to control the river. According to Doug Megargle, Regional Fisheries Manager with the Idaho Department of Fish and Game, “the channel could have swung up to half a mile or more in one direction or the other,” prior to development in the floodplain. Dr. Bruce Lium of American Water Resources, a Hailey river contractor for more than 30 years, says that the spring flooding accomplished great things in terms of restoring the river’s natural functions. “The biggest benefit,” Lium notes, “is that the carrying capacity of the river has definitely increased.” According to Lium, the 2006 flood “probably has taken some of the channelization away.”

Removal of in-stream wood (logjams, cottonwood trees, rootwads, and woody debris jams that pile up in the river) that has occurred over the years negatively impacted fish habitat and flooding patterns. In-stream wood introduces bugs and other nutrients into the system which fish and other aquatic creatures need. It also plays a key role in slowing high flows and trapping sediment. As late as 1984, the Army Corps of Engineers spearheaded a “clear and snag” project which cleared 10 miles of the river channel in the Big Wood River of the many logs, rootwads, and woody debris jams that gave the river its name. Removal of the instream wood compromised the ability of the river to form pools crucial to healthy fish habitat.

Restoring the river that runs through it.

In recent years, efforts by local organizations such as Wood River Land Trust have improved the health of the river. Wood River Land Trust has completed restoration projects at different sites along the river including Lions Park, Bullion Bridge, and Riverside Pond in Hailey, as well as Boxcar Bend just south of Ketchum.

The restoration projects were designed to bring back a more natural function to the river and floodplain. The projects shared some common goals: restoring a more natural grade to and stabilizing riverbanks, allowing the river to flood and function more naturally by reconnecting it to its floodplain and preventing unnaturally large amounts of sediment from entering the flood waters.

In May and early June of 2006, the efforts paid dividends, assisting with flood control, protecting water quality, and preventing erosion. Plantings along the restored slopes at Bullion Bridge, Lions Park, and Riverside Pond stabilized the banks during high water and prevented sediment from entering the river. These restored banks demonstrate the greater stability of natural banks in comparison to banks where vegetation has been removed and shows one way to address future flooding with greater success.

At Riverside Pond, the pond itself acted as a sediment cache as sheet flooding entered the pond, slowed, settled out dirt and sediment, and returned to the river. During the flooding, the City of Hailey cleared the entry point for the pond to enable the maximum volume of sheet flow to enter the pond and protect neighbors from flooding. >>>


The value of the floodplain.

Floodplain areas are critical for water quality, providing overhanging vegetation that keeps water cool for fish during the summer months, and crucial areas for water to spread out during flood events. The river needs these areas and so do the fish. Doug Megargle explains that in floodplain areas sediment can settle out as water slows down. “If [the river] can’t expand into side channels first and it can’t expand into the floodplain successfully, the river will scour and recruit an abnormally high sediment load,” he says. Increased sediment harms fish populations when it is disturbed or deposited in areas that fish need for spawning; trout need clean gravel on which to lay their eggs. Too much sediment in the channel can also impact the river’s interaction with the groundwater below the channel.

When the river is cut off from the floodplain, it cannot settle sediment outside of the main channel. This leads to unnaturally high amounts of sediment settling in the floodway which contributes to increased flood volume and velocity. Allowing the river as much room to move as possible and allowing the river to sheet flood and settle out sediment is critical to spreading out its energy and keeping flood volumes—and damage from flooding—at lower levels. Rather than constricting the river’s tendency to meander, the river should be allowed to move and spread out, particularly during flood events.

Lessons learned for future flood fighting.

Property owners living in the floodplain are understandably concerned with flooding. This year starkly revealed the risks of living in the floodplain. Historically, individuals have applied for stream alteration permits that allow them to riprap or otherwise alter the river to protect their property from flooding. Yet, there are limits to this individual approach. Riprap often transfers flooding to a neighbor downstream, across the river, or both.

Dr. Lium says that the wood deposited into the Big Wood this year is good not just for the river and its fish, but for landowners as well. “We should leave it in the system,” says Lium. Lium not only thinks it should remain in the system, but he hopes to use cottonwood logs in bank stabilization projects. To further stabilize the banks, landowners should add plantings along banks on top of the log work. In some cases, simply restoring native plants along river banks will help preserve banks during flooding and slow the flows.

What does the future hold?

This year, the Big Wood River lived up to its name. River anglers and other enthusiasts will experience a river that harkens back to a by-gone era of pools and logjams too numerous to count. As Fish and Game’s Megargle notes, this year it is “time to learn a new river. What was a run may not have water in it anymore; pools will be where they have never been before with large thanks to woody debris.” Heading out to a favorite fishing hole and finding it gone is one way to learn. But if we are to fully embrace the lessons of the new river, then we must learn how to live with the new river, as well as the process by which it was re-born. A natural force that plays an important role in the health of the communities in the Wood River Valley, the river can be tricky to contain. The wisest approach is to yield to it as much as we can, because the power of the river only increases as we try to confine it.

Carrie Norton, Code Compliance Specialist with Blaine County, sees a limit to the property-by-property approach to flood response. “If everybody works together on it, better solutions for more people can be reached.” Norton believes floodplain residents need to “come together and craft a vision for the river, beginning at the neighborhood level. First and foremost, we have to allow the floodplain to function.” While this doesn’t mean letting bridges wash out or endangering homes, it does mean allowing sheet flooding to enter the floodplain, using more natural techniques to stabilize banks and protecting homes over landscaping.

Approaches include re-vegetating areas along the river that have been covered in grassy lawn with native shrubs with good root systems, such as red osier dogwood and willows and re-thinking when and how to construct more intensive bank stabilization projects. Norton also raises an important point for local communities to consider when planning for future flooding: costs to the public. Planning for restoration at the neighborhood level can eliminate costly, last-minute dikes and riprap. These types of projects constructed amidst flooding keep the river from sheet flooding and transfer damages from flooding downstream.

Private landowners, local governments, non-profit organizations, and state and federal agencies will all play a role in protecting the health of the river. A coordinated vision that brings all the players together and allows the most natural functions of the floodplain to continue will benefit all—and potentially cost communities less in flood-fighting dollars over time. Leaving wood in the river is one vital step. Local ordinances setting development away from the river’s edge will play a key role, as will future restoration projects designed to improve the function of the floodplain. Private landowners can have a major impact on the health of the river by taking a cooperative and multi-faceted approach to protecting their property. And when waters rise, homeowners should protect their structures, while allowing the sheet flooding to occur.

Working together, residents of the Valley’s communities can ensure the health of the river while safeguarding private property. Letting the river change, grow, and reach its greatest potential will be the measure by which we can judge our success as neighbors who live on and love the Big Wood River.

This article appears in the Winter 2007 Issue of Sun Valley Magazine.