324 happ. STREAM-CHANNEL CONTROL [Ch. 18 



The growth of meanders is often stopped by development of shorter 

 chute channels across the bars formed on the inside of the bends. 

 Chutes may develop because the resistance to flow around the length- 

 ening bend becomes greater than across the bar, or because changes 

 in alignment caused by channel shifting upstream tend to direct flow 

 across the bar inside the bend. 



In natural streams many bends and meander loops are abandoned 

 because of cutoffs resulting from adjacent bends migrating gradually 

 into each other, or from channel avulsions across the necks between 

 adjacent or even more distant bends during periods of overbank flood- 

 ing. Most natural cutoffs of the first type are apparently due to rela- 

 tively resistant deposits in the alluvium, which check the migration of 

 one bend until it is overtaken by the downstream movement of an- 

 other (Hearn, 1932; Matthes, 1941; Fisk, 1944). During a cutoff of 

 this type on the Missouri River in 1880, a deep bed of clay held a neck 

 only 8 feet wide for 24 hours between adjoining bends 4% miles apart 

 by river distance (Church, 1881, p. 1623). The 1891 cutoff shown in 

 Fig. 2 was a Missouri River avulsion of the second type, across a nar- 

 row meander neck, by erosion of overbank flood water (Fox, 1892, 

 p. 3289) . A partial avulsion of the same type occurred on the Missouri 

 River at Saline City Bend in 1915, as shown in Fig. 1, but the river 

 diversion was not complete and the chute channel was subsequently 

 shut off by bank-protection works and a rock-fill dam (Mclndoe, 1916, 

 p. 2649) . The cutoff of several bends or meanders by a single avulsion 

 seems characteristic of aggrading rivers, such as the Middle Rio 

 Grande above Elephant Butte Reservoir (Happ, 1948, p. 1197 and 

 Fig. 2, Plate I). Meander cutoffs or shortening by chute developments 

 reduce channel lengths and increase slopes and hence are generally 

 beneficial for reducing flood heights or improving drainage; but they 

 may cause much local damage by channel shifting and bank erosion, 

 and resulting unstable bed conditions that may interfere with navi- 

 gation. 



Natural streams flowing on alluvial beds normally develop alternat- 

 ing series of deep and relatively narrow pools, typically formed along 

 the concave sides of bends, and shallow, wider reaches between bends 

 where the main current crosses the channel diagonally from the lower 

 end of one pool to the upper end of the next. During high flows the 

 pools or bends tend to scour deeper, while the crossing bars are built 

 higher by sediment deposition, although deposition does not equal rise 

 in stage and hence water depth increases on the bars. When the stage 

 falls, there is erosion from the top of the crossing bars and some filling 

 in the pools; but, as low-stage activity is less effective, the general 



