The bridge will be closed for the foreseeable future, said Whatcom County Sheriff Bill Elfo.
Whatcom County Public Works crews set up a detour around the area.
The nearest Nooksack River crossing is via Marine Drive to the south.
The Main Street bridge in Ferndale and the I-5 river crossing are the nearest routes across the river north of Slater Road.
Slater Road is the primary route for hundreds of workers to reach the BP Cherry Point and Phillips 66 refineries and the Alcoa Intalco Works aluminum smelter, as well as the Lummi Nation's Silver Reef Casino, Lummi Reservation lands and Sandy Point.
The 9:30 am crash also took down power lines in the area.
This isn't the first time an equipment collision has closed the Slater bridge - a previous crash shut the crossing for months while crews repaired significant damage that weakened the bridge's structure.