The Tank Vessel Mobiloil, while en route from Ferndale, Washington to Portland, Oregon,
suffered a steering failure at 0005 on March 19, 1984
which resulted in the grounding of the vessel near Warrior Rock
in the Columbia River approximately 10 miles downstream from Portland, Oregon.
Punctures and gashes ruptured the number 1-5 starboard cargo tanks
releasing 3,925 barrels of heavy residual oil, industrial fuel oil, and No. 6 fuel oil.
The vessel remained grounded perpendicular to the current until March 26,
when it was refloated and escorted to dry dock in Portland.
At the time of the spill the wind was from the south
and the river current velocities were 1.5-3.0 knots.
The incident occurred in a turbulent area of the river that experiences some tidal influence.
No. 6 fuel oil is a heavy product with an API gravity that ranges from 7 to 14.
Spilled oil was distributed on the surface of the river,
throughout the water column, and on the river bottom.
The shoreline was not impacted as severely as anticipated.
Areas thought to be thick oil were actually sheen.
The only black oil observed on the surface occurred as small tar balls
that were commonly found clinging to debris.
The sinking nature of the oil made estimates of the spill size difficult.
Initial Mobil and USCG estimates of 1,000 barrels were revised
after additional information suggested that the majority of the oil
had been incorporated into the water column and in the river bed.