REMOVE

1. trucks
2. workers
3. tools
4. rock

WHAT

1. Grand Coulee Dam (dam) (1)
2. Visual Works (hierarchy n … (1)
3. banks (landforms) (1)
4. construction sites (1)
5. cranes (equipment) (1)

More 

WHERE

1. Grand Coulee, Washington (1)

WHO

1. Clyfford Still; Still, Cl … (1)
2. Still, Clyfford (1)

WHEN

1. 1936 (1)
2. 1937 (1)
3. circa 1936 to 1937 (1)