Water Flow
WATER FLOW
Speed: At the headwaters of the Mississippi, the average surface speed of the water is near 1.2 miles per hour – roughly one-third as fast as people walk. At New Orleans, on 2/24/2003, the speed of the river was 3 miles per hour.
Length: River length is a difficult measurement to pin down because the river channel is constantly changing. For example, staff at Itasca State Park, the Mississippi’s headwaters, say the Mississippi is 2,552 miles long. The US Geologic Survey has published a number of 2,300 miles (3,705 kilometers), the EPA says it is 2,320 miles long, and the Mississippi National River and Recreation Area maintains its length at 2,350 miles.
Width: At Lake Itasca, the river is between 20-30 feet wide. The Mississippi is widest just downstream from its confluence with the Missouri River (near Alton, Il.) where it is nearly 1 mile across. [Note: In the first mile of the river, just out from Lake Itasca, there were places where I could touch both banks of the river with my canoe paddle, certainly less than 20 feet in width — G.H.]
Depth: At its headwaters, the Mississippi is less than 3 feet deep. The river’s deepest section is between Governor Nicholls Wharf and Algiers Point in New Orleans where it is 200 feet deep.
Volume: At Lake Itasca, the average flow rate is 6 cubic feet per second.
At Upper St. Anthony’s Falls, the northernmost Lock and Dam, the average flow rate is 12,000 cubic ft/second.
At New Orleans, the average flow rate is 600,000 cubic feet per second.
There are 7.489 gallons of water in a cubic foot. One cubic foot of water weighs 65.4 pounds. A 48 foot semi-truck trailer is a 3,600 cu. ft. container. At Itasca, it would take 10 minutes for one semi-trailer of water to flow out of the lake into the Mississippi. At St. Anthony Falls, the equivalent of 3 semi-trailers full of water go over the falls every second. At New Orleans, the equivalent of 166 semi-trailers of water flow past Algiers Point each second.
Elevation: The elevation of the Mississippi at Lake Itasca is 1,475 feet above sea level. It drops to 0 feet above sea level at the Gulf of Mexico. More than half of that drop in elevation occurs within the state of Minnesota.
Sediment Load: The Mississippi carries an average of 436,000 tons of sediment each day. Over the course of a year, it moves an average of 159 million tons of sediment. Averages have ranged from 1,576,000 tons per day in 1951 to 219,000 in 1988.
Watershed Area: The Mississippi River Basin or Watershed drains 41% of continental United States. Thirty-one states and 2 Canadian provinces are included in the watershed. The total area drained by the watershed is between 1.2 and 1.8 million square miles.
The course and position of the mouth of the river into the gulf has changed at least seven times in the past 10,000 years (Heller, 2006). The river gradually builds up its floodplain until it is much higher than the surrounding land. Then the river would find a new, lower, shorter path to the ocean by cutting through one of its banks and eroding out a new bed for itself.
If the river were to continue shifting positions every time it found a shorter, steeper route to the Gulf, many port cities would need to be periodically relocated. This happened often before the Army Corps of Engineers built up and monitored the levees, preventing the river from changing course (Twain, 1967). The Corps built the Old River control structure during the 1950s to prevent the Mississippi from shifting its flow entirely into the steeper, more direct route to the Gulf via the Atchafalaya river bed. The Mississippi currently empties about 30% of its water into the Atchafalaya (Sparks, 2006). The Mississippi bed is much higher than the Atchafalaya at Old River, so without the control structure, the Mississippi would send 100% of its flow down the Atchafalaya. This would leave all cities currently located along the Mississippi below Old River completely dry, and flood most of the cities currently located along the Atchafalaya.
Spur dikes, or wing dams, are used to control the width and depth of a channel of flowing water (Hendrickson, 1999). They are porous barriers, usually constructed of quarried rock laid down during times of low water. The dike extends only partway out into the channel, where it slows down the water that passes through and around it, causing the river to deposit sediment and further build up the dike. If the opposite bank is armored, preventing the river from maintaining its original velocity and hydraulic radius, the spur dike serves to narrow and deepen the channel, thus increasing the river’s velocity. A higher velocity also increases the river’s carrying capacity. This means that it will pick up more sediment, and if it cannot do this by eroding the banks outward, it will cut the bed deeper.