The term discharge is often used interchangeably with the term flow. River discharge is the measure of volume of water passing a given cross-section. Historically, discharge measurements are calculated from water level using a stage-discharge curve, or what is commonly called, a rating table. The rating table was developed by taking successive discharge measurements at different heights of water level, also known as gauge height. Early measurements of discharge utilized current meters, which are mechanical devices that measure the rotation of cups (or similar), over a period of time to get water velocity. The discharge measurement is constructed by taking the mechanical meter across the entire width of the river at given depths of water. Newer methods of river dicharge utilize sonic doppler signals, incoporated in ADCP's, which are quickly becomoing the new standard for accurate river discharge measurement.