A canal pound (from impound), reach, or level (American usage), is the stretch of level water impounded between two canal locks. Canal pounds can vary in length from the non-existent, where two or more immediately adjacent locks form a lock staircase, to many kilometres/miles. The longest canal pound in the United Kingdom is between the stop lock on the Trent and Mersey Canal at Preston Brook (Dutton Stop Lock No 76) and the start of the Leeds and Liverpool Canal near Leigh (Poolstock Bottom Lock No 2), a distance of . Another long pound is on the Kennet and Avon Canal between Wootton Rivers Bottom Lock and Caen Hill top lock. The longest level on the Erie Canal in New York was the 60 mile level (actually ) between Henrietta and Lockport. History Pounds came into being with the development of pound locks to replace the earlier flash locks. A key feature of pound locks was that the intervening level between locks remained largely constant, as opposed to the variable levels created by the opening of flash locks. On some American canals, some pounds acquired nicknames due to the mileage between locks, e.g. the "Eight Mile Level" on the Chesapeake and Ohio Canal was between Riley's Lock (Lock 24) and Edward's Ferry (Lock 25), a distance of about . Engineering considerations Making a channel that would not leak water is not easily done. Gravel soil or light soil will leak for a few years until it settles and hardens. Puddle or clay would be used to line the channel, but that was not foolproof. The Union Canal and the Schuylkill canal where the canal prism (the shape of the canal as seen in cross section) was blasted through limestone leaked so badly, they had to line the channel with timber. The Chesapeake and Ohio canal had frequent cave-ins due to limestone sinkholes near Shepherdstown, near Two Locks above Dam No. 4, around Four Locks, Big pool, and Roundtop Hill near Dam No. 6. requiring expensive repairs. After some years, the canal bed would settle and harden, and puddling would no longer be needed. Water supply One main concern to the engineer was the canal's water supply. Pumping would contribute greatly to operating expense, especially at the summit level, hence gravity fed water is preferred. Feeder canals could be to in length, if needed. The canal bed would have to slope