CHANNEL FILLS OF ALL SCALES FROM THE MISSISSIPPI RIVER ALLUVIAL VALLEY

Filled abandoned channels are common in meandering stream systems. Scales include single channels of the entire Mississippi River, secondary channels where the Mississippi channel has split around in-channel bars, stable secondary channels that only receive a portion of the discharge but may persist for several thousand years, distributary channels that siphon a considerable portion of the Mississippi River flow onto the flood plain for 1-2 ky, crevasse channels that siphon a small portion of the Mississippi River discharge on the flood plain, probably for <0.5 ky , and batture channels that connect the main Mississippi channel with an oxbow lake for up to a few thousand years. These abandoned channels can be divided into three groups based on fill style.

The first group includes rapidly abandoned channels such as neck cutoffs. Channels which are rapidly abandoned, deposit bars at the entrance to the abandoned meander, probably in a few decades. These cutoff bars are wedge-shaped, medium-sand deposits that may extend several kilometers into the abandoned channel. After these bars effectively block coarse-sediment input, the remaining abandoned channel fills in a few thousand years with silt and clay transported through small batture channels into the oxbow lake.

The second group includes slowly abandoned channels such as chute cutoffs, distributary channels, and crevasse channels. Discharge into these channels is slowly decreased but the channel being abandoned retains an effective connection with the active channel for some time. Sediment brought into the channel being abandoned continues to be relatively coarse bed-load as well as suspended load. Massive, fine-grained, suspended-load sediment accretes along the channel margins causing the channel to become narrower. Medium to fine sand eventually fills the central portion of the channel causing the channel to become shallower and eventually fill.

The last group includes slowly abandoned channels such as secondary and batture channels. Discharge into these channels is decreased but continues for an extended period of time. Sediment load into the abandoned channel is generally restricted to the suspended load. The channel fills with thin-bedded silt and very fine sand with common organics. Batture-channel fill is a facies within a larger neck-cutoff channel fill.