A visit to a marijuana farm in Willow Creek, the heart of northern California’s so-called Emerald Triangle feels like strolling through an orchard. At 16 feet high and eight feet around, its 99 plants are too overloaded with cannabis buds to stand on their own. Instead each plant has an aluminum cage for support.

Welcome to America’s “pot basket.” The U.S. Drug Enforcement Administration estimates 60 percent of cannabis consumed nationwide is grown in California. According to the Department of Justice, the bulk of that comes from the three upstate counties of the Emerald Triangle: Mendocino, Humboldt and Trinity. Conditions here are said to be perfect for outdoor marijuana cultivation. But that has proved to be a very mixed blessing for the region, bringing with it a litany of environmental disturbances to local waterways and wildlife. Creek diversions threaten fish habitat and spur toxic algal blooms. Road building and clear-cuts erode soil and cloud streams. Deep within, illegal “guerilla grows” pepper forestlands with banned rodent poisons that are intended to eradicate crop pests but are also fatal to other mammals.

On November 8 voters in four states—Massachusetts, Maine, California and Nevada—legalized recreational marijuana. These states join Colorado, Washington, Oregon and Alaska, along with the District of Columbia, where one can already legally buy the drug for recreational use. Will this expanded market mean more environmental damage? Or will legalization pave the way for sounder regulation?

In 1996 California legalized marijuana for medical use, providing the first legal space for pot cultivation since the federal government’s blanket ban on the crop some 60 years before. As grow operations in the state flourished, California Department of Fish and Wildlife biologist Scott Bauer analyzed satellite imagery to examine the impact of cultivation on water levels in four Emerald Triangle watersheds. His study, published in PLoS ONE in 2015, found that in three of the four watersheds, “water demand for marijuana cultivation exceeds stream flow during the low-flow [summer] periods.”

The real problem is not marijuana’s overall water consumption, which still falls far short of California staples like walnuts or almonds, explains environmental scientist Van Butsic of the University of California, Berkeley. Rather it is an issue of where and when pot is grown. Analyzing aerial imagery of 4,428 grow sites in 60 Humboldt county watersheds, Butsic found that one in 20 grow sites sat within 100 meters of fish habitat and one in five were located on steep land with a slope of 17 degrees or more. “The problem is that cannabis is being grown in the headwaters, and much of the watering is happening in the summer,” Butsic says.

If that arrangement goes on unchecked, U.C. Berkeley ecologist Mary Power warns, summer plantations could transform local rivers from cool and “salmon-sustaining” to systems full of toxic cyanobacteria. Over eons of evolution native salmon species have adapted to “deluge or drought” conditions, she says. But the double whammy of climate change and water extraction could prove to be a game-changer.

Power spelled out the unprecedented stresses in a 2015 conference paper focused on the Eel River that flows through Mendocino and southern Humboldt. She and her team found riverbed-scouring floods in winter, followed by dry, low-flow conditions in summer, led to warm, stagnant, barely connected pools of water. That is bad news for salmon, but ideal for early summer algal blooms. The algae then rot, creating an oxygen-deficient paradise for toxic cyanobacteria, which have been implicated in the poisoning deaths of 11 dogs along the Eel River since 2002.

Dogs are not the only terrestrial creatures endangered by the grow operations. Between 2008 and 2013 Mourad Gabriel, then a doctoral candidate at the University of California, Davis, Veterinary Genetics Lab, carried out a study of the American fisher, a small carnivorous mammal that is a candidate for the endangered species list. He wanted to suss out the threats to fisher populations in northern California. So he radio-tagged fishers from Trinity County’s Hoopa Valley Reservation and public lands near Yosemite National Park to track their movements. Between 2006 and 2011, 58 of the fishers Gabriel and his team tracked turned up dead. Gabriel studied the necropsies and found that 46 of the animals had been exposed to anticoagulant rodenticides—rat poisons that block liver enzymes, which enable blood clotting. Without the enzyme the exposed mammals bled to death from flesh wounds.

The finding puzzled Gabriel at first, because rat poison is more common in agricultural and urban settings than in remote forests. But then he started visiting the remnants of guerilla grows that had been busted under the guidance of lawmen such as Omar Brown, head of the Narcotics Division at the Trinity County Sheriff’s Office. “We have found [anticoagulant rodenticides] carbofuron on grows in the national forest,” Brown reports. “These are neurotoxin-laced pesticides that have been banned in the U.S. since 2011. And even for allowed pesticides, we’ve found instances where trespass grows are using them in illegally large quantities.” The poisons hit female fishers particularly hard, because the early, pest-prone phase of marijuana cultivation coincides with the fishers’ nesting season, when pregnant females are actively foraging.

Gabriel, now director of the Integral Ecology Research Center based in Humboldt County, says other states may be dealing with rodenticides, water diversions and other problems from guerilla grows, too. “The climate in Colorado, Oregon and Washington is conducive for marijuana cultivation,” he observes. But “there just isn’t the scientific data to prove whether other states have these problems because there has not been research funding put towards answering these questions.”

In California headwater ecosystems could get a reprieve if a greatly expanded legalized pot industry moves to the Central Valley, where production could take place indoors and costs would be less. In pot-growing pioneer states like Colorado or Washington much of the production has moved indoors, where temperatures can be more closely managed. But other factors may hinder that move. “Bud and pest problems are always worse indoors, which biases farmers toward a chemically intensive regime,” says Marie Peterson of Downriver Consulting, a Weaverville, Calif.–based firm that helps growers fill out the paperwork for state and county permits as well as assesses water management plans for their plantations. And besides, the Central Valley already suffers from prolonged drought.

Of the eight states that legalized the cultivation of recreational marijuana, only Oregon and California allow outdoor grows. But regulating open-air pot plantations in these states remains challenging, even though legal operations for medical marijuana have been around since 1998 and 1996, respectively. In 2015 California passed the Medical Marijuana Regulation and Safety Act, which calls on the state’s departments of Food and Agriculture, Pesticide Regulation, and Fish and Wildlife, along with the state’s Water Board—to oversee environmental impacts of the industry. The board came up with a list of requirements for a marijuana plantation water permit, which in turn became a necessary condition for a license to grow medical pot in any of the three Emerald Triangle counties. Counties have until January 2018 to decide whether to create similar stipulations for recreational marijuana growing permits.

Butsic is optimistic about a more regulated future for the marijuana industry in California. “I think five years from now things will be more sustainable. Permitting shows growers that the state is interested in water use and their crop.”