REMOVE

1. rubbish
2. rivers
3. plants
4. construction sites
5. sky
6. buildings (structures)

WHAT

1. Visual Works (hierarchy n … (1)
2. banks (landforms) (1)
3. black-and-white film (1)
4. bushes (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. ca. 1936 - 1937 (1)