At the Crossroads of Salmon Streams and Marijuana Cultivation, by Larry DeRidder

Cliff HartAlerts

It’s been over six months since California legalized recreational marijuana cultivation and use. Some published reports indicate that California produces something on the order of two-thirds of the country’s supply, and our three-county area produces something on the order of two-thirds of California’s supply. If you do the math, that means nearly half of all of the marijuana consumed in the country comes from our region. Nationwide, government eradication efforts from 2008 to 2012 show that 53% to 74% of all the plants eradicated by law enforcement were in California, which further bolsters the contention that marijuana farming is a significant economic driver, both statewide and locally. The New York Times on September 9, 2017 reported an estimated 32,000 marijuana farmers in the Emerald Triangle. The Times Standard on January 11, 2018 reported that 15,000 of them are in Humboldt County, of which only about 2,000 have made any attempt to “go legal”. This brings up the question of what the impact of all this agricultural activity is having on our salmon spawning streams.


Agricultural impacts to salmonid streams can include (1) groundwater diversion, (2) chemical and sediment runoff into waterways, (3) streambed alteration or blockage, an (4) removal of canopy vegetation and general disruption of the regional ecology. Looking back at the last 150 years or so, these are many of the same concerns which led to government mandated reductions in logging and gravel extraction, though in those cases not until after substantial ecological damage had taken place. In many cases we’re still trying to recover from the damage caused by prior economic activities.


In the March 2015 PLOS One (a scientific journal) there was an article titled “Impacts of Surface Water Diversions for Marijuana Cultivation on Aquatic Habitat in Four Northwestern California Watersheds”. Bear in mind that field work done in 2013 would observe marijuana cultivation after the passage of Proposition 15 (medical marijuana), but long before the legalization of adult recreational use. Thus, it’s likely that the cultivation of marijuana in each of the four studied watersheds is higher today than at the time of the study. Most agricultural water use diversion studies have concentrated on large-scale diversions, such as the Potter Valley Hydroelectric Project. This study looked at the cumulative impact of many small diversions. If you would like to see the original article, it can be found at

Impacts of Surface Water Diversions for Marijuana …

Marijuana (Cannabis sativa L.) cultivation has proliferated in northwestern California since at least the mid-1990s. The environmental impacts associated with marijuana cultivation appear substantial, yet have been difficult to quantify, in part because cultivation is clandestine and often occurs on …



The primary purpose of the study was to estimate the impact of the first of our four listed agricultural effects on streams. To arrive at their results, the authors used low altitude aerial imagery in 2013 to estimate the number of parcels visibly undergoing cultivation and the amount of ground covered with marijuana plants. They then accompanied law enforcement visits to more than 40 previously photographed sites. This allowed them to compare the photographic evidence with actual plant counts on the ground, which permitted an estimate of the total number of plants present. The number of plants times the daily water demand per plant resulted in an estimate of the total water required to support marijuana cultivation in each watershed. Then they compared those numbers with the amount of water naturally present.


The four study watersheds were Upper Redwood Creek, Salmon Creek and Redwood Creek South (all in Humboldt County), and Outlet Creek (Mendocino County). These four watercourses were selected based on the presence of sensitive aquatic species such as coho salmon, sufficient marijuana cultivation to provide realistic statistical analysis of results, and which either contained water gauges, or had nearby similar streams with water gauges.


Water demand per plant was based on calculations from the 2010 Humboldt County Outdoor Medical Cannabis Ordinance draft, which states that marijuana plants on average require 22.7 liters (6 gallons) per plant per day during the growing season. Calculated water availability was the annual seven-day low flow, a measurement often used to define the low flow of a stream, and which is defined as the lowest value of mean discharge computed over any seven consecutive days of the year. Locally the lowest flows are likely to be late summer prior to our first serious rainfall.


In many rural watersheds, the primary domestic water source is small surface water diversions, which must be registered with the State Water Resources Control Board, and cleared with California’s Department of Fish and Wildlife. Of the watersheds visited, half to as many as 94% of the water diversions noted were neither legal nor registered.


Marijuana cultivation sites were widespread in all four of the studied watersheds. Generally speaking the grows detected from the air were clustered together rather than randomly spaced. The studied portion of Upper Redwood Creek was estimated to have 79 parcels under cultivation, Salmon Creek 189 parcels, Redwood Creek South 187 parcels, and Outlet Creek watershed 441 parcels. Minimum estimated plant totals ranged from 23,000 to 32,000 per watershed. Thus water consumption estimates ranged from 138,000 to 191,000 gallons per day. Calculations assumed that all of the cultivation sites were drawing water. This estimate could not, of course, calculate and include water use from sites which were not identified from the air, nor did they include water diversions for household use.


It was recognized that some years are “wetter” or “drier” than average, so a range of values resulted. Final results indicated that in three of the four watersheds, water demand for marijuana cultivation could exceed what is naturally supplied by surface water alone for some times of the year. Upper Redwood Creek fared best in this analysis, where it was estimated that between 2% and 23% of the stream was diverted. Salmon Creek, Redwood Creek South and Outlet Creek watersheds reflected diversion rates ranging from 34% to over 100%.


The late summer minimum stream flow estimates in Salmon Creek, Redwood Creek South and Outlet Creek are so low that in some cases as few as four standard sized 10 gpm (gallons per minute) pumps could dewater the mainstem stream if operated at the same time. Given that the largest water demand for marijuana cultivation would be at the end of the growing season, and before we can expect any significant rain, the effects on stream residents can reasonably be expected to be lethal.


Just how we as a society are to balance the needs of our local streams and their inhabitants with marijuana cultivation will be an ongoing legislative and enforcement issue. If law enforcement closed down one illegal marijuana grow every day Monday through Friday, May through October, that would be something on the order of 250 operations per year. Yet with an estimated 13,000 illegal grows just in Humboldt County, that level of enforcement activity would only clear about 2% of the problem per year. Just how to best protect our local salmon spawning streams is something we will have to determine, but if salmon and steelhead are to survive and prosper, a long-term solution must be found and enforced.