The Delta area has four transmission towers that provide television and radio reception for residents of Northern California, and three rival some of the tallest structures in the world: Twin Cities Tower at 2000 feet carrying channels 40 and 6, Walnut Grove Tower at 1549 feet carrying channels 10, 13, and 3 backup, McCormick Tower at 2007 feet carrying channels 3 and 31, and Sacramento Joint Tower at 200 feet carrying channels 10, 13, and 58. Built in 1946, the Delta Cross Channel is a man-made channel less than a mile long that connects the Sacramento River with Snodgrass Slough and the Mokelumne River system. At the Sacramento River end is a pair of hydraulically controlled gates with a usual low-tide overhead clearance of about 8-1/2 feet when open; although often called locks, they are not locks. The gates may be closed for flood control, improvement of water quality, or protection of migrating fish, and Bureau of Reclamation operating procedures call for closure when Sacramento River flow reaches the 20,000 to 25,000 cfs range. Opening or closing the gates generally takes 30 minutes to one hour. The deep channel has a bottom width of 210 feet and a capacity of 3,500 cubic feet per second, allowing low-overhead-clearance boats to use a shortcut between the two waterway systems instead of traveling many miles by way of Georgiana Slough and the North Fork of the Mokelumne River.