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Marijuana Grows Are Draining Some California Streams, Study Finds

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One of California's most lucrative crops may be drying up streams | Photo: Mark/Flickr/Creative Commons License

The thriving, not always legal marijuana cultivation industry is having a significant impact on Northern California's streams and rivers, according to a study by state and federal wildlife biologists. In some cases, pot farms may be taking enough water out of local streams to drain them dry, and that could be having devastating effects on local salmon and trout, along with a number of other wildlife species.

The study, published last week in the journal PLOS ONE, surveyed outdoor grows and greenhouses in three watersheds in Mendocino and Humboldt counties that eventually drain into the Eel River, as well as a drainage basin tributary to Humboldt County's Redwood Creek.

By estimating the number of plants in each watershed and multiplying by each plant's typical water use, the researchers were able to estimate how much water might be diverted from local streams to keep the plants alive -- and in drought years like this one, that turns out to be more water than some creeks hold.

The study, Impacts of Surface Water Diversions for Marijuana Cultivation on Aquatic Habitat in Four Northwestern California Watersheds, was conducted by California Department of Fish and Wildlife staff Scott Bauer, Jennifer Olson, Adam Cockrill, Michael van Hattem, Linda Miller, and Gordon Leppig, along with Margaret Tauzer of the National Marine Fisheries Service.

The summation in the study's abstract is blunt:

Estimates from the other study watersheds indicate that water demand for marijuana cultivation exceeds streamflow during the low-flow period. In the most impacted study watersheds, diminished streamflow is likely to have lethal or sub-lethal effects on state-and federally-listed salmon and steelhead trout and to cause further decline of sensitive amphibian species.

The watersheds surveyed were the South Fork Eel River tributaries Outlet, Salmon, and Redwood creeks, along with the upper reaches of a second Redwood Creek which flows through the Redwood State and National Parks before reaching the ocean at Orick. Researchers used aerial surveillance to map and count both outdoor plants and greenhouses, then calculated a typical plant spacing inside those greenhouses after studying 32 similar greenhouses that had been secured by law enforcement.

The study's authors arrived at an estimate of 112,320 plants for all four study watersheds. Multiplying by 22.7 liters of water per plant per day, an estimate provided by the Humboldt Growers' Association, the authors calculated the likely water demand of all those plants at 2,549,890 liters per day -- just under 674,000 gallons a day taken from four smallish streams.

During drought years, the researchers calculated, that would be more demand on the water in those streams than three of the four streams actually held, meaning that diversion for pot grows could actually make those three streams dry up.

And that, of course, would be devastation to local aquatic species that can't go without water. The watersheds the team surveyed are home to federally threatened runs of coho and chinook salmon and steelhead, as well as officially designated State Species of Concern, including western pond turtles, coastal cutthroat trout, tailed frogs, the northern red-legged and foothill yellow-legged frog, southern torrent salamanders, and the mollusk western pearlshell , which is critically imperiled in California.

"Our results indicate that the high water demand from marijuana cultivation in these watersheds could significantly impact aquatic- and riparian-dependent species," write the authors. "In the Pacific Coast Ecoregion, 60 percent of amphibian species, 16 percent of reptiles, 34 percent of birds, and 12 percent of mammals can be classified as riparian obligates, demonstrating the wide range of taxa that potentially would be affected by diminished stream flows."

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