It's 4/20, international cannabis appreciation day.  What better time to consider the environmental impacts of marijuana prohibition?  I say prohibition, and not cultivation, because there has already been significant attention paid to the environmental impacts of cultivation.  Mostly these criticisms focus on the anarchic nature of marijuana farming culture, and the extent to which these lawless operations despoil the environment.  Withdrawing water without permits, clearing forested areas, and using fertilizers that run-off into nearby streams are among the impacts, and I'm sure there's truth to that.  But it's worth asking why that is taking place, and what role marijuana laws are playing.  This seems to be one school of thought in response to state measures to regulate marijuana farming:

The marijuana industry has long been the province of lawbreakers, and it seems unlikely that those who have been conducting their business without any legal oversight would readily adopt measures to protect the...environment from the impact of their actions.

Recent history suggests otherwise.  In 2010 Mendocino County and local growers developed a plant registration system that helped farmers and the county comply with environmental laws.  It showed promise until federal marijuana prohibition laws broke up the partnership:

Almost 100 growers participated, but the program was shut down in early 2012, after federal agents raided one of the grows and US Attorney Melinda Haag hinted that she might just take the county to court. Later that year, a federal grand jury subpoenaed the county's zip tie records. 

The environmental impacts of marijuana cultivation might be significant, but they are made worse by forcing otherwise law-abiding farmers out of the regulatory system.  While it's easy to speculate that most marijuana farmers don't have water use permits, it's more difficult to offer a solution that doesn't run afoul of state or federal prohibition laws.  In many cases when marijuana industry entrepreneurs have tried to comply with local or state laws, those efforts back-fired by making it easier for federal prosecutors to identify and prosecute them.  

All that might be coming to an end thanks to an obscure amendment and an ongoing case in the 9th Circuit Court of Appeals.