- Chilliwack with The Collectors and friends, tonight and tomorrow night at Richmond's River Rock Show Theatre at 8 p.m. Tickets: $50 through www.ticketmaster.ca.

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A riot seemed inevitable. In October 1970, Greenpeace supporters and music lovers had gathered in Vancouver for the concert that launched the grassroots environmental movement.

The show, Amchitka, was intended to raise funds to send a ship to an area of the Aleutians to protest United States nuclear weapons testing. James Taylor, Joni Mitchell, Phil Ochs and the Lower Mainland's own Chilliwack were among those on the bill.

When Ochs took the stage, he referenced another political issue facing the nation -- the October Crisis. Political unrest in Quebec led to the kidnapping of two government officials by the Front de Libération du Québec, resulting in then Prime Minister Pierre Trudeau calling on the War Measures Act to deploy Canadian Forces members on the ground.

"There was a dangerous kind of feeling happening, that things could erupt, that there could be violence," says Bill Henderson, leader of Chilliwack, thinking back on the day.

When his band took the stage, they had an idea. The rockers opted to improvise, playing long, beautiful notes, stretching them out, then gradually building the intensity of the music.

"This possible riotous environment had been transformed to one where people were at peace and were having a great time," he says. "And we didn't do that by addressing it literally . . . we just played notes. And most of the time, it was that kind of thing. We'd pick up on the vibe that was in the room and that's what we would do. That kind of approach to music is very, very rare."

That special quality, connection to their fans, belief in the power of music as well as the strength of members' relationships has contributed to Chilliwack celebrating its 40th anniversary this year. To mark the milestone, the band is playing two shows, tonight and tomorrow, at Richmond's River Rock Show Theatre at 8 p.m. The two performances will be filmed for an upcoming CBC Television documentary that will also feature interviews and archival footage of the band over the course of its history.

Henderson, a Salt Spring Island resident, has been with the band from Day One and views the reunion as a means of paying tribute to their supporters.

"It's definitely for the fans," he says. "If you know Chilliwack songs, both "Raino" and "Fly at Night" quite directly talk about the audience and how much they mean to us. . . . And I don't just mean because they come out and buy tickets and help us to earn a living. It's got to do with their involvement in the music, in the show itself."

The band has gone through different "eras" over the years and the early 1970s is what Henderson refers to as the "first" era of Chilliwack, which at that time was primarily focused on improvisation and jamming.

"The improvisation was always coming from not just our own thoughts and ideas, but also the room, how the room felt, what the vibe was out there, what was going on," he says. "We would feed that into the music. Sometimes very literally and sometimes more subtly."

While Amchitka was a more subtle example of this approach, 1970's Winnipeg, Man., Man-Pop Festival, headlined by Led Zeppelin, is telling of their more direct approach. When they took the stage, they noticed a group of people using a blanket as a make-shift trampoline, bouncing people up and down.

"Most of the audience was turning around, looking at them and here we were up there playing music. . . . So we started singing about 'Hey, over here' and we made up a tune. . . . They all realized that the band up there wasn't just playing a set list, the band up there was playing with their minds," he laughs. "The band up there was aware of what was going on with those people and wanted to be involved. We inserted ourselves into their afternoon and they responded. They came back to us."

The positive relationships among band members over the years has also helped with Chilliwack's longevity, despite inevitable lineup changes.

"We were good friends at the time and when we did part we didn't part on bad terms," says Henderson. "We basically kept pretty civilized about it."

There have been 26 members of Chilliwack over the years and many will be taking the stage for the performance, including Claire Lawrence and Howie Vickers from The Collectors, the 1960s band that ultimately gave rise to Chilliwack.

Also taking the stage will be Roy Forbes, who plays with Henderson and Shari Ulrich in UHF and who was a member of Chilliwack for a year; and bassist Ab Bryant.

Former members who've passed away will be honoured at the performance as well. Howard Froese's son Tyson will fill in for him on the acoustic solo he made famous on "Baby Blue." Henderson is particularly excited about the successful end to a search for Brian MacLeod's old guitar. After MacLeod died, his fender strat with a cherry sunburst finish, Floyd Rose bridge and locking nut, exchanged hands a number of times. The band issued a public outcry for anyone knowing of its whereabouts and Wednesday, got word that it was safe with a man in Calgary. Alta., who funnily enough, is a big fan of the band. The current owner attended his first Chilliwack concert at age 14.

The guitar is en route to Vancouver and will be incorporated into tonight's show.Some new faces are also on the reunion show bill, including Henderson's two talented daughters Saffron and Camille. Both singers, Saffron, focuses on voiceover work and Camille is a former member of the West End Girls and at one time sang backup for Sarah McLachlan.

Henderson is incredibly excited to take the stage tonight. "It's a very unique thing, it's not going to happen again. This is, I'm sure, the last time. I doubt that the 50th will happen. I think this is the time, you know, it's kind of like you better get out and see this because it's not going to happen again."

That said, Chilliwack has a packed summer tour schedule and Henderson's work with UHF and solo projects are still on the go. He's also working on a book with Martin Melhuish, tracing the history of West Coast music.

"I love to play music in front of people, to have a show and play for people and with people, and that's about as good as it gets for me," he says.

emcphee@nsnews.com