The article needs to make this clearer - it could be read as meaning that the creek water surface area was 5000 acres - see The historical Islais Creek, at the time the largest body of water within San Francisco covering an area of 5,000 acres (7.812 sq mi; 20.234 km2)[4], ... I also note the ref is "nearly 5000 acres", not exactly 5000 acres> I think the figures used are too exact then and it should be something more like "the watershed was almost 5000 acres in area..."