A NATIONAL PLAN FOE AMERICAN FORESTRY 413 



STREAM FLOW AND FLOOD PROBLEMS OF THE DRAINAGE 



As has been pointed out, major overflows in the Mississippi Delta 

 have their source outside the boundaries of the drainage basin. In 

 the lower Mississippi Basin the outstanding flood problem directly 

 attributable to run-off from within the drainage occurs in the Yazoo 

 Delta. This is an alluvial flood plain 180 miles long and up to 65 

 miles wide, lying between the Yazpo and Mississippi Rivers north of 

 their confluence in western Mississippi. Here major floods occur 

 periodically every few years, frequently during the winter months 

 after heavy and prolonged rains. They originate in north central 

 Mississippi on the watersheds of upland tributaries, and are the result 

 of surface run-off from the uplands poured quickly and in large 

 volumes on a region of nearly level topography and poor drainage. 

 These overflows often cover hundreds of thousands of acres and do 

 widespread damage. The most recent Yazoo flood occurred during 

 the period December to February, 1931-32. This overflow, the 

 greatest on record, inundated some 600,000 acres and imposed great 

 hardships. According to estimates of the United States Weather 

 Bureau, it resulted in property damage of almost $1,500,000, not 

 including the damage to crops and livestock. 



Floods of less magnitude occur on the bottomlands of other streams 

 draining the uplands of the lower Mississippi Basin. These streams 

 have a low gradient and are rather sluggish. Some have developed 

 flood plains several miles in width and these are flooded almost 

 annually during periods of heavy rainfall. On these bottomlands, 

 the most disastrous floods occur during the summer months when 

 heavy rains produce overflows that inundate growing crops. 



Although the normal discharge of these streams is slow, discharges 

 at flood are turbulent. Even at normal flow the water is strongly 

 discolored and carries large volumes of silt. In addition, flood waters 

 transport enormous quantities of heavier materials and, during periods 

 of overflow, often cover productive alluvial farm lands with an infer- 

 tile blanket of sand and gravel. The channels of creeks and other 

 small tributaries are frequently filled with such detritus, thereby 

 causing more frequent and extensive overflows on adjacent bottom- 

 lands. The sediment carried by the headwater drainages becomes of 

 Greater economic concern when it reaches navigable streams and is 

 eposited in the channels, thus necessitating costly dredging opera- 

 tions. In 1931, such work in the Memphis district of the Mississippi 

 River cost nearly $800,000. For the lower river as a whole it has 

 cost to date some $19,000,000 to remove soil eroded from the water- 

 sheds of the Mississippi River system. 



The streams of the lower Mississippi Basin are perennial and flow 

 throughout the year. The flow, however, is not uniform and is par- 

 ticularly erratic for those streams draining only upland watersheds. 

 Irregularities in flow are indicated by stream gage records of the 

 United States Geological Survey which show, for the Coldwater 

 River, a ratio of maximum to minimum discharge of 725 : 1 ; for the 

 Yallobusha River, 486 : 1 ; and for the Yazoo River 43:1. The erratic 

 behavior of these streams in common with others in the basin is 

 evidenced during periods of heavy and prolonged rainfall when rises 



