California has endured intense droughts before. The state has also been swamped many times by intense rain and massive floods.

But this winter could bring an unusual event in which a strong El Niño weather pattern follows an extreme dry spell. This, say scientists and officials, could be a recipe for especially damaging floods that cause erosion, landslides and water pollution.

Biologist Kevin Shaffer, at the California Department of Fish and Wildlife, says soils have become compacted, much like a dried-up sponge, in the course of the four-year drought. That means the first heavy rain, rather than soaking into the earth, will race downhill. It will scour away soil and debris as it gathers in gullies and rushes seaward, potentially tearing apart the landscape.

A great deal of vegetation has also withered away through the drought, and previously verdant meadows and mountain slopes now look like hills on the moon – drier and dustier than they would be at the end of most California summers. Without living root tangles to hold the soil together, the ground in such places will be vulnerable to slumping and sliding as the ground becomes saturated, Shaffer says.

This could spell problems for struggling populations of salmon and steelhead, which have already been negatively impacted in most watersheds by human-induced erosion.

In Sonoma County, Don McEnhill says soil sampling has recently measured some of the lowest moisture levels ever recorded in the region, causing native, drought-tolerant plants like coyote brush to wilt and die.

McEnhill, the executive director of the group Russian Riverkeeper, hopes to see several mild rainstorms arriving before any real drenchers do. This would have the effect of opening up the porous soils without generating floods, making the ground able to absorb water again – much the way a dried sponge must first be hydrated before it can function.

Then, he hopes the skies portion out the winter’s rain slow and steady.

“We could handle 60 or 70 days of moderate rainfall,” says McEnhill. “What hurts is getting two or three big storms in a row.”

Mudslides have already hit some burn areas. A modest storm in mid-October caused debris flows on areas burned in Rough Fire in the Sierra National Forest. Some roads were buried in mud. As a result, Forest Service officials say many areas that would normally remain open to the public will stay closed all winter for safety reasons.

Fish need water, of course, and Shaffer says heavy rainfall in the months ahead will certainly be good for species that spawn in large, robust rivers less prone to becoming explosive torrents following rainstorms, like Chinook salmon and white sturgeon. In California, these species spawn in their greatest numbers in the Sacramento and Klamath rivers. In fact, biologists who track the fish know that white sturgeon tend not to spawn at all except in the wettest years. The last time white sturgeon had an exceptional spawning event was in 2006, and a torrential El Niño winter may be exactly what the biggest freshwater fish in North America needs.

But fish that inhabit smaller coastal streams immediately surrounded by steep slopes and high mountains will be more vulnerable to flooding and erosion. Too much rain at once will turn trickling creeks and dwindling pools into deadly torrents that could wash juvenile salmon and steelhead out to sea, killing them. Landslides in headwater creeks could smother gravel beds where adult fish spawn each winter.

This is roughly what happened in 1964 on the North Coast’s Eel River, when a gargantuan December flood filled the river valley with about four times the volume of the Mississippi River. Scott Greacen, executive director of Friends of the Eel River, says heavy logging in the years prior to the big flood made the landscape vulnerable to heavy erosion.

As a result, the flood filled the river with muck, he says, burying not just that autumn’s generation of Chinook salmon eggs but also smothering the spawning beds entirely, making them unsuitable as habitat in the near future.

“There was very low survival of that year’s fish, and successive years didn’t have much better luck,” Greacen says. He says the Eel River’s Chinook have never bounced back to their pre-1964 levels.

Intensive conversion of forest to marijuana plantations in the past several years has similarly set the stage for another potential disaster.

“The best-case scenario this winter would be getting lots of light rain,” Greacen says. “The nightmare scenario is that a really substantial atmospheric river event connects the hot water in the Pacific with the jet stream heading over us.”

Atmospheric rivers, also commonly called “pineapple express” storms, are unique to the Pacific Coast. They occur when tropical moisture is drawn by the jet stream into a narrow column of wind that then aims drenching rain at a relatively small area of land. Such storms have historically been responsible for California’s worst floods.

The result of such an event, Greacen warns, could be collapsing hillsides and roads, and “rivers that we’ve been recovering for years filled up again with sediment – wrecked again.”

Some tiny streams – especially those south of San Francisco, from the Santa Cruz Mountains to Santa Barbara – harbor runs of salmon and steelhead already on the brink of vanishing, thanks to environmental issues like water pollution and habitat loss. If abrupt floods strike in these watersheds, entire year-classes of fish could be lost, Shaffer says. Even Monday’s moderate rainfall prompted flash flood warnings for parts of Northern California from the National Weather Service.

Abrupt, heavy rains will cause more than flooding and landslides this winter. McEnhill says we can expect a burst of water pollution under such conditions, since rapid runoff will wash nine months of accumulated vehicle exhaust particulates, motor oil drippings, dog feces, trash and other debris into streams.

“All that pollution is going to greet the salmon as they migrate upstream,” McEnhill says.

Inland parts of the state have also been made vulnerable to heavy rainfall thanks, in part, to the drought. Forest fires have scorched hundreds of thousands of acres in California this year. Vast numbers of charred dead trees stand in fire zones that burned so hot, in some cases, that all organic matter in the soil was destroyed.

The 2014 King Fire did precisely this.

“So that leaves nothing in the soil to hold it together,” says Ross Branch, media relations director with the Placer County Water Agency.

The U.S. Forest Service has estimated that the King Fire, which burned 98,000 acres, left about 300,000 tons of loose topsoil vulnerable to being washed away during rain. If it does wash away, this material will accumulate in downstream reservoirs, says Branch, and may require expensive removal operations in the future to offset the loss of water storage space in reservoirs.

But for all the concerns about how heavy rain may impact a landscape parched by drought, biologist Jacob Katz thinks any rain must be wholly welcomed this winter. Katz, Central California program director with the group California Trout, says California’s plants and animals have lived through eons of wet-dry cycles. He isn’t sold on the notion that too much rain will cause significant harm to fish and wildlife.

“When you enter the fifth year of drought, sensationalizing big floods doesn’t make sense to me,” he says.

Katz agrees there could be costly impacts of heavy rainfall this winter, and he agrees the landscape is currently less able to absorb surplus water than it has been in the past.

“But that’s not because of the drought or the transition over [from dry conditions to wet],” he says. “Big floods happen, and they can be bad for fish, mostly because we have deeply altered river systems where you’ve basically accelerated the drainage.”

Katz says it would be more appropriate to blame any environmental consequences on the way river systems were overhauled in the 20th century. The major rivers were rerouted into narrow channels and bracketed with levees that separate the moving water from natural floodplains. This has accelerated storm runoff during rain events, and it shortens the duration of time in which water remains in a watershed. The result, Katz says, is a landscape that floods more easily during storms and dries out more rapidly afterward.

Other scientists agree that human activity has reduced the environment’s ability to absorb the impacts of extreme weather swings. Tom Bruns, a U.C. Berkeley professor of plant and microbial biology, says decades of fire suppression in California’s forests have allowed underbrush to grow in unnatural, unsustainable densities. This makes forests more susceptible to extremely hot burns, like the King Fire. It also heightens competition for water during drought, weakening even very large, old trees that could otherwise have withstood years of dry conditions. The result is that the entire forest begins to fail.

Another potential problem this winter is proliferation of sudden oak death, the blight that has ravaged coastal oak woodlands. Bruns explains that the pathogen spreads in bursts when moisture is combined with warmth. This winter is predicted to be not just wet, but warm – perfect conditions for the disease to expand, Bruns says.

McEnhill, at Russian Riverkeeper, says homeowners can help curb runoff and fight flooding – at least in urban areas – by simply modifying their rooftop and landscaping arrangements. He suggests people install rainwater capture systems and use the water later in the year for irrigation. Gardens, too, should be leveled and planted with vegetation that keeps the water from draining away immediately.

Farmers, he says, should encourage the growth of cover crops while applying compost to increase organic matter in the soil, which helps hold water in place.

“People just need to slow water down so that it sinks into the ground and enters creeks later in the season instead of all at one time,” McEnhill says.

Alastair Bland is a freelance journalist based in San Francisco. He can be reached via email at [email protected].

Top image: Rock, mud and debris remain on a closed road in the Sierra National Forest following an afternoon rain storm in Oct. 2015.