Illegal outdoor marijuana grows on California's North Coast are sapping 18 million gallons of water a year from an Eel River tributary, according to the L.A. Times. That water consumption is threatening a salmon species that California has spent "millions of dollars to recover." In Humboldt County, growers are using rat poison mixed with human food to kill bears and fishers (a type of weasel), both of which animals threaten clandestine growing operations. Threats to salmon and weasels aren't the only problem: "Farmers have illegally mowed down timber, graded mountaintops flat for sprawling greenhouses, dispersed poisons and pesticides, drained streams and polluted watersheds."
Illegal indoor grows present their own environmental problems, as discussed in a 2011 study from the Lawrence Berkeley National Laboratory (emphasis mine):
Specific energy uses include high-intensity lighting, dehumidification to remove water vapor, space heating during non-illuminated periods and drying, irrigation water preheating, generation of CO2 by burning fossil fuel, and ventilation and air-conditioning to remove waste heat. Substantial energy inefficiencies arise from air cleaning, noise and odor suppression, and inefficient electric generators used to avoid conspicuous utility bills.
The emergent industry of indoor Cannabis production results in prodigious energy use, costs, and greenhouse-gas pollution. Large-scale industrialized and highly energy-intensive indoor cultivation of cannabis is driven by criminalization, pursuit of security, and the desire for greater process control and yields.
To its credit, the L.A. Times manages to pin-point why outdoor growers are loathe to run more environmentally friendly operations: "Because marijuana is unregulated in California and illegal under federal law, most growers still operate in the shadows." In this case, literal shadows: Growing pot in a dense forest is wiser than growing it in plain sight. The Lawrence Berkeley report also notes the incentives driving energy-consuming indoor grows: "air cleaning, noise and odor suppression, and inefficient electric generators used to avoid conspicuous utility bills."
Remove the threat of prosecution, and marijuana growers would have no incentive to obscure their operations in dense, hard-to-reach forests. They wouldn't need to mow down protected land to plant, and they wouldn't need to poison bears and fishers. They might also be more likely to comply with permitting processes for water use.
Similarly, fully legal indoor grow operations could abandon fossil fuel generators, and use less energy attempting to eliminate plant odor. With the knowledge that police would prosecute theft and burglary of their plants, indoor growers might also spend less on security infrastructure.