Director Michael Peterson and writer Steven Hawley aren’t afraid to be unsubtle with their ecological documentary “Dammed to Extinction,” which was screened in a sneak preview in the Wheeler Theater at Fort Worden State Park May 4.

Peterson says he sought to personalize the plight of the dwindling killer whale population of the Pacific Coast, getting audiences emotionally invested in the animals before exploring the reasons for their declining numbers.

The film manages to succeed by employing a relatively no-frills approach, combining on-location video with montages of news clips and historic footage, laid over a series of talking-head interviews with a baker’s dozen of people who care about, and have studied up on, the fate of the Pacific orcas.

The film didactically arranges these testimonials in support of the film-maker’s case: the whales are failing to reproduce enough to sustain their population because Snake River dams in Washington and Idaho have cut off salmon, the major orca food source, from their spawning grounds.

The film’s testimonials assert not only that the removal of the four dams along the Snake River could more than double the current salmon population - whose surviving members are often too old to adjust, by the time they transition from fresh to salt water — but also that removal of the dams would not represent a significant loss to the power supply of the Pacific Northwest.

While the Columbia and Snake Rivers have four major dams each, the Columbia River is deeper and colder than the Snake River. The Snake’s relatively shallow waters pose greater dangers of overheating or being preyed upon for salmon, which already struggle to climb fish ladders that were the first concession dam operators adopted when it became clear dams were killing off northwest salmon runs.

Peterson’s skill as a director, honed from previous environmental documentaries and work on visual effects in films like Independence Day and Contact, reveals itself here in his juxtaposition of interviews, attesting to the pristine conditions of the Snake River as a salmon habitat, with footage of its mountains, forests and waters that complement such claims.

Carrie Chapman Nightwalker Schuster, an elder with the Palouse Tribe, is captured onscreen growing emotional as she recounts how the damming of the river affected her people, but her account is not the only means by which the film personalizes the impact of the damming. News clips recount how a Pacific Northwest orca mother carried her dead calf for an unprecedented 17 days of mourning last summer.

Given the number of Pacific Northwest state legislators whose defenses of those dams are likewise captured on-screen through news clips, I’d be interested in their responses to this film’s arguments. As a documentarian, Peterson makes a coherent, compelling case with “Dammed to Extinction,” which he hastened to inform his Port Townsend audience is still undergoing edits.