In a large conurbation a record was kept of alternate supplies of water that could be obtained other than fire hydrants situated on the water mains supply. This was in case of water mains being fractured mainly due bombing. These would include any rivers or canals that were in the vicinity. This was where a portable dam or reservoir was made for fire fighting purposes usually indicated by the letters EWS - emergency water supplies.
In small towns and rural areas the were small dams and recors were kept if wells, outdoor swimming pools as well as streams and rivers. Harbours were also included but tide tables were needed for tidal areas.
