Sonoma Rainfall

Muddy Waters

David Licht

A River Romps Through It: Debris-choked flood waters spilled over River Road in 1995 after storms killed one person and caused several million dollars in property loss, destroying or damaging 1,500 homes. River residents worry that steep-slope planting of vineyards, development, and siltation could compound problems.

Climatologists predict record rainfall this year as Russian River residents brace for nature’s wrath–and what some say is the backwash of man’s fallacies

By Christopher Weir

WITH THE RESURGENTEl Niño already being blamed for virtually everything from hurricanes to the Oakland Raiders’ dismal record, one thing seems certain: The rogue phenomenon will unleash a troublesome dose of flood-instigating rainstorms upon Sonoma County this winter. Or will it?

As the local daily proclaimed in a recent subhead, “The North Coast could be in for severe weather–or it may escape unscathed.” Never has something so speculative been met with such morbid anticipation by local, regional, and national news media. But if El Niño does, indeed, live up to its billing, then there will likely be hell to pay on the local front. And since everyone is in such a speculative mood these days, perhaps it’s time to dredge up the unpopular question: Why? That is, why do flood events in Sonoma County appear to be intensifying even under apparently normal weather conditions?

Some suggest that this perception is just misperception. The historical record, after all, reveals that the Russian River has crested above flood stage 40 times over the past 41 years. Meanwhile, as more people have moved into the floodplains, the human toll has become more acute. Throw in the recent 7-year drought, and suddenly the watershed’s natural inclinations seem the stuff of anomalous catastrophe. The flood issue, however, is not that simple. Because while historical records show peak flows and flood frequencies, they don’t reveal the Russian River watershed’s spatial and behavioral changes over time for a given volume of water.

“Nobody’s really looking at how flood levels are changing according to the size of the floods,” says Laurel Marcus, a local hydrologist. “For example, is a 10-year flood creating more problems than it used to? There are all sorts of variables, so that’s not a simple question.”

Those variables are receiving increased attention in the wake of last January’s floods, which many county residents felt were excessive, rising faster than in recent memory and in the absence of saturating precipitation, at least relative to the rainfall amounts. And of those variables–everything from watershed development to allegedly misguided flood-control projects–siltation remains the most provocative.

“If you’ve got a river channel with a certain capacity and silt material comes out of the water and settles in the bottom of that channel, it’s going to decrease the capacity of that channel,” says Dennis Wilson, an independent civil engineer based in Forestville. “When this happens over many years, that decrease is significant. Is this happening or not? Common sense tells me that, yes, it is happening, because that silt doesn’t just disappear, and it all doesn’t flow into the Pacific Ocean.”

Wilson says he’s not armed with hard data, just intuition, anecdotal reports from riverfront residences, and personal experience. For example, he has seen survey monuments no more than 20 years old buried beneath eight feet of silt in the Steelhead Beach area. And he suggests that the accumulation of municipal development and certain agricultural practices–especially steep-slope planting of vineyards on former forestland–is the primary culprit.

“When I see this degree of siltation, it just seems obvious to me that it’s going to cause flooding,” he says. Others, however, are not convinced that siltation is a major flood factor in the main stem of the Russian River.

“You can certainly say that there’s more sediment running through the system,” says Joan Vilms, president of Friends of the Russian River. “There’s no question about that. Whether it’s being built up in some places and depleted in others, or whether it’s a consistent buildup, I can’t say. . . . Sediment is somewhat like sand dunes, in that it’s dynamic. It’s really hard to get a handle on it. You can’t really say just because somebody’s fence is covered up that the whole plain is now at that level.”

Noting that the Russian River system is “disturbed and altered,” Marcus says that determining the capacity impacts of siltation would be an exceedingly complex proposition. Silt is not only “highly mobile,” but also very responsive to both the natural and the unnatural dynamics of the river system.

“When you pave an area, you create so much more runoff . . . that you can gully a creek and move out more silt than is coming in,” she says. “And you can have an increased silt load below a grading site that may sit there one winter but be completely flushed downstream and out to sea the next winter.”

Nevertheless, Wilson’s intuitions are being confirmed in some river tributaries and floodplains, including the Laguna de Santa Rosa. While maintaining that siltation is not a flood issue in the river’s main stem, George Hicks, deputy chief engineer at the Sonoma County Water Agency, adds that “the Laguna de Santa Rosa has been silting up, and the source is in the upper watershed areas.”

The Laguna de Santa Rosa is crucial to local flood mitigation and holds as much water attenuation capacity as Warm Springs Dam. The water agency is currently conducting a study to assess the amount and impacts of siltation in the laguna.

Erosion and siltation are also playing supporting roles to another element of the flood equation: accelerated storm-water runoff from developed lands and drainage systems. While the degree to which accelerated runoff is exacerbating flood events remains a matter of debate, it’s clear that continued watershed development and related erosion-control measures are increasing the rapidity and volume of water flowing from the watershed and into the river system.

THE CONVERSION of sloping wildlands to vineyards is the current flashpoint in the development angle, with the 500-acre Gallo project on Westside Road drawing the most fire. With sediment and silt increasingly recognized as significant Russian River pollutants causing harm to steelhead trout populations, agricultural developments–especially hillside vineyards–are compelled to establish efficient drainage systems to minimize the erosional impacts of storm-water runoff.

But while such drainage reduces sediment loads, it intensifies the runoff that is already accelerated by the removal of native vegetation. According to Dr. Martin Griffin, local conservationist and owner of the Hop Kiln Winery, the Gallo project will have a serious impact on the water loads in nearby tributaries. “The water drops into these big pipes at the end of each vineyard row on the slopes, and then it goes right into Porter Creek and into the river,” he says. “It’s an industrial drainage system that’s equivalent to what’s used in cities.”

Included in Gallo’s plans is the cutting and conversion of 174 forest acres. In such cases, Griffin says, “You’ve lost the vegetation that slows the impact of the water. . . . Trees drip water for days, for weeks, and the soil absorbs it through root systems. Then it gradually moves downhill, and that’s what keeps the river and creeks full in the summer: the slow release of water from the watershed.”

Last week, Gallo announced plans to hold off on the project for a year. Though the winery has not yet withdrawn its application to cut down the trees, doing so would make anEIR legally unnecessary.

He adds, “Gallo is just one of the biggest conversion problems. It’s happening all up and down the river, on the ridges. Every time they clear an acre up on the steep hillsides, they’re just increasing the flood potential for Guerneville.”

Vilms agrees, saying, “Every time you drain an area faster, it does two things: It eliminates the ability of that land to hold water, and it races that water more quickly downstream.”

Griffin and Vilms both maintain that while gravel mining has created in-stream pits that ostensibly increase the river’s capacity, the resultant channelization of the banks has simply heightened the pace at which water is delivered downstream.

That floodplain development and subsequent stream channelization is the most insidious factor in the changing flood dynamic, Vilms adds. The channelizing of Santa Rosa Creek, for example, “was successful in eliminating the flooding from that area, but by doing so, you just caused it someplace else. It’s a zero-sum game. There’s only so much water and only so much land, and if you build on one floodplain and eliminate its capacity to hold water, then you’re taking that water and delivering it downstream to somebody else who already has their own load to deal with.

“We should be finding places to retain water rather than accelerating drainage.”

Others, however, are not convinced that development is having a measurable impact on local flood events. “You have 1,500 square miles in the Russian River watershed,” says Mike Thompson, a civil engineer with the Sonoma County Water Agency. “And when you look at the developed areas, it’s really minor with regard to the entire watershed.”

Hicks suggests that development in the lower watershed may even be mitigating flood impacts. “You want to retain water in the upper watershed and get water out of the lower watershed,” he says. Lake Sonoma and Lake Mendocino enhance water retention in the upper regions, while development can accelerate drainage in the lower regions, he says. “So you take the volume and you’re spreading it out. Your volume is the same, but the impact on people and property is significantly reduced if you can spread it out.”

He adds, “Generally speaking, the development in Sonoma County is beneficial, or at least not additive” to local flood scenarios.

ACCORDING to Marcus, however, development and urbanization manipulate the volume, timing, and intensity of runoff to a degree that has yet to be fully ascertained. “This is a million-acre watershed, with most of it draining into the river by the time you’re in Guerneville,” she says. “So you have all this land upstream that’s affecting the flood levels down there.

“Now the question that’s not really been answered is, ‘Given different scenarios of buildout in the urban core, what’s the difference in the flow for different sizes of floods?'”

She notes that while a single hillside vineyard project or residential development will not have an appreciable impact on flood potentials, the cumulative effects of such activities should not be underestimated. “There are a lot of questions that relate urban growth to flood levels,” she says. “But to my knowledge, they have not been looked at. There are no storm-water retention ordinances in Santa Rosa, but a lot of places with flood problems require developers to retain storm water on-site or work with the city [on retention], rather than dumping it into the nearest conduit and giving it to somebody else.”

Why hasn’t a comprehensive analysis of possible development-related flood impacts been conducted in Sonoma County? Because the jurisdictional and financial implications have yet to be fully explored.

For example, the Sonoma County Water Agency’s authority in the watershed is surprisingly minimal. “Our business is maintaining the inventory of flood-control channels and some natural creeks that we’ve been empowered to maintain,” Hicks says. “And I’d estimate that it’s between 5 and 10 percent of the county’s waterways.”

Hicks adds that the water agency has no jurisdiction over development patterns and infrastructures.

Meanwhile, county agencies and municipalities have yet to establish a cohesive flood-control relationship. “The county desperately needs an overall flood-management program,” Griffin says. “It has flood-control programs for specific cities, which just pass their floodwaters on as fast as they can get them out of there by draining them into the Russian River and on down to Guerneville.”

Hicks, however, cautions against extrapolating broad flood-management lessons from highly localized scenarios. “With flood control, there’s the small scale and the large scale. Things tend to apply differently, and more intuitively, on a small scale.”

Adds Thompson, referring to last January’s floods, “People looked at the record and said, ‘We didn’t have much rain in Santa Rosa but we got this horrendous flood.’ But a lot of that was due to the nature of the storm.” That storm, he says, was concentrated in the northern watershed, yielding a tremendous amount of water that was subsequently dispatched downstream.

“Each storm has its own personality,” Thompson says, “and it’s going to react differently depending on the intensity of the rainfall, where it rains, and the timing of it. It’s a very complex system.”

And in major flood events, the difference between pavement and wildlands is narrowed as supersaturation takes hold.

“In 1986, it rained for several days,” Hicks says. “It got to the point that there was a sheet of water even on undeveloped lands. The longer it rains, the closer it gets to mimicking developed runoff conditions.”

THE WATER AGENCY analysis in the wake of the 1986 flood determined that the water peak would have been six to nine feet higher without the watershed’s two major dams, Hicks add.

But Vilms says that “the flood-control aspects of Warm Springs Dam are greatly overrated” and that the best approach to flood mitigation is the preservation of natural river ecosystems. “Allowing streams to flush themselves and maintain their own floodplains is really the only solution,” she insists. “Any other solution is temporary, and ultimately nature will override it.”

If developmental forces are indeed exacerbating flood problems for downstream communities, then mitigating actions should be taken. But the only way to determine overall impacts is to do the math, a process that has yet to be undertaken on a broad scale.

“You have to look at changes in peak flow,” Marcus says. “For so much rainfall in the drainage basin, is the water higher in Guerneville vs. the same size storm previously? Then you need to look at the different tributaries and see how they might have changed in their flow characteristics. These are not simple questions, but they’re the kind that need to be grappled with in a way that focuses on data, not finger pointing.

“Let’s look at the problem in an objective fashion and try to really resolve whether this is a real perception or not.”

From the Nov. 6-12, 1997 issue of the Sonoma County Independent.

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