The TMEP would draw heavily on supply from the Alberta tar sands in Canada. These bitumen deposits require a lot of effort to extract, and process before the oil can be sent to conventional refineries. Most of the current extraction process takes place in open-pit mines, with massive machinery scraping up the tarry sandstone and moving it to facilities for processing. The heavy crude is found mixed with sand, clay, and water, which must be removed, then the heavy crude must be "upgraded" to reduce viscosity and improve quality (The Atlantic, Greenpeace). This extraction process has a heavy carbon footprint and has a a devastating effect on land, producing massive amounts of toxic byproducts. Studies show oil sands crude emits more greenhouse gases than conventional crude oil. Pipeline expansion enables the expansion of this toxic extraction.

The existing and proposed pipelines ship diluted bitumen through an extremely sensitive environmental region. The tankers have to pass through a very narrow channel of shallow water to reach the open sea, posing a major threat to an endangered population of orcas (the iconic “killer whales” beloved across the world, and culturally significant for the Tsleil-Waututh people).

The original Trans Mountain Pipeline has a controversial history of oil spills; in the period 1961-2016 there were 81 reported incidents. A July 2017 Greenpeace report concluded that oil spilled into bodies of water is difficult to fully clean up, posing serious risks to human health and the environment,. Diluted bitumen transported from Canada’s tar sands represents a particular threat to water resources along the routes of proposed pipelines.

The Trans Mountain Pipeline expansion poses a grave threat to Indigenous people, as described in this letter written by several NGOs and groups of Indigenous people to 28 banks who were, at the time, involved in financing the project. The parallels with the struggle over the Dakota Access Pipeline are clear, leading to questions about whether the Trans Mountain conflict may become the “Standing Rock of the North.”

The Trans Mountain project does not have the Free, Prior and Informed Consent (FPIC) of the Indigenous people it will affect. First Nations groups that would be directly impacted by the route and port terminal are fighting the project in the courts and leading heated protests on the ground. As of July 2019, Tsleil-Waututh Nation, Squamish Nation, Ts’elxweyeqw tribes, Shxw’owhamel Nation, Coldwater Indian Band and Stk’emlupsemc te Secwepemc Nation were involved in multiple legal challenges to the pipeline’s construction. and direct action campaigns have been ongoing. The Tsleil-Waututh established a “Watch House” on their territory to monitor and resist construction; Secwepemc women have established the “Tiny House Warriors,” small, mobile houses built to reoccupy Secwepemc territory and prevent pipeline construction. Detailed reports, such as Standing Rock of the North and Assessment of the Trans Mountain Pipeline and Tanker Expansion Proposal outline, in detail, crucial elements of Indigenous opposition to the project.

As noted by the Treaty Alliance Against the Tar Sands, the expansion of the Canadian tar sands would lead to increased destruction and poisoning of lands, waters and air on Indigenous territory.  There is widespread opposition to tar sands expansion among these groups, who are committed to fighting the pipelines which make such expansion profitable.