Sounding the river--determining its depth--used to be done with poles, or a lead line, or by a captain's best guess. Ripples on the water surface may indicate shallows; conversely, "still waters run deep." The outside of a bend in the river generally runs deeper than the inside. "Mark twain" was the call of a deckhand informing the captain how much line was in the water (therefore the depth) when the weighted end struck bottom. Too often, these approximations failed to give a captain the tools he needed for safe navigation. Accidental groundings were the most common cause of riverboat damage before the rivers were charted and before electronic echo-sounders were invented.

The pictured survey boat is "mapping" the riverbottom with a gang of depth sounders protruding into the water on both sides. The precise location of each depth reading is determined by a continuous GPS signal which is correlated by computer with the sounders.

The resulting 3-D map of the bottom will be used to identify silted areas in or near the shipping channel that need to be dredged. The data will also be used to update charts and to relocate markers. Commercial tug captains rely upon these markers (buoys and posts) to keep their deep draft vessels from going aground. As with all navigation aids, these markers can be erroneous, and should be used in context with other available information in determining the safest course.