20 (1) Notwithstanding anything in these Regulations, where, because of channel obstructions, a casualty, the weather, ice conditions, water levels or other unforeseen or temporary circumstances, compliance with these Regulations would be impossible, impracticable or unsafe or would cause a risk of pollution, the Regional Director General, in the case of Canadian waters, or the District Commander or the Captain of the Port, in the case of United States’ waters, may temporarily instruct ships to proceed in a certain manner or by a certain route, or to anchor in a certain place, or prohibit ships from proceeding or anchoring except as specified in lieu of or in addition to any provisions of these Regulations.