Lillooet River's not dammed, but the Meager itself is and there's a lake growing quickly upstream of Capricorn Cr.
The question is, when it blows out, how far will the wave go downstream?
The historic Devastator slides of 1930s and 1970s did not send floodwaters all the way downstream to the current location of Pemberton, but there is plenty of sedimentological evidence that older (prehistoric) landslides DID send floodwaves as far as the current Pemberton town site, and indeed, all the way to Lillooet Lake.