The morning news is showing pictures of the area. It doesn't appear to be a stream or river; simply high water built up both sides of the highway, and over the highway. The "fix" seems to be waiting for it to evaporate or soak into the ground.
According to
http://www.drivebc.ca/, the government site for road conditions:
Flooding Highway 37 Both directions - The Hwy is closed in both directions 12 km north of Dease River Bridge because of flooding. (80 km north of Dease Lake). Water levels remain high. No estimated time of opening. Detour not available - alternate route via Alaska Hwy. Next update at 10:00 am May 31st. Updated on Fri May 31 at 6:29 am PDT.----------------------------
If one is northbound on the Cassiar, it would take some pondering to decide whether to turn back or wait it out.
From Dease Lake it is only a couple of hundred kilometres, maybe a two hour drive, to the Alaska Highway.
But to backtrack south on the Cassiar, then east on Hwy 16 to Prince George, then north to Dawson Creek, and finally northward on the Alaska Highway would be about 2,350 km, likely a four day drive.