Ch. 1] RIVERS AND DELTAS 25 



sell, 1885). The activity of organisms is a potent influence in the 

 deposition of calcium carbonate in lakes. 



Rivers and Deltas 



Many different types of environments of deposition are found on 

 land. For the world as a whole, terrestrial deposits are less plentiful 

 than marine deposits; but in places, notably in Wyoming and Colo- 

 rado, thick non-marine deposits have accumulated in the past. These 

 sediments are mainly the result of stream action. The essential re- 

 quirement is the formation of a place of deposition, particularly a flat 

 surface upon which the rivers can deposit their load at times when 

 they get out of their channels. 



Most deposits laid down by running water on land are poorly sorted 

 and poorly stratified, except in channels where currents are relatively 

 strong and uniform. Most river and flood-plain deposits vary in 

 thickness and are cross-bedded. Alternations between coarse and fine 

 deposits are common. 



River deposits are characterized by cut-and-fill action during flood. 

 (See Chapter 18.) When the discharge is great and the velocity of the 

 water is high, the bed of the stream is scoured to a greater depth than 

 during normal periods of flow. As the flood abates and the stream 

 returns to normal levels, the depth of scour diminishes and the bed 

 fills up to its customary position. Large streams may be scoured 

 more than 50 feet, as is indicated by boards found during constructions 

 of piers, or by the undermining of deep piers during floods. In con- 

 structing caissons in rivers, geologists and engineers should endeavor 

 to determine the depth to which the river scours during floods and de- 

 sign foundations accordingly. 



At time of flood, streams rise above their channel and spread across 

 the adjoining flat land, leaving deposits of silt and sand (Fisk, 1944). 

 Deposits of this type ordinarily are not well sorted and tend to vary 

 in grain size and thickness from place to place both longitudinally and 

 laterally across the flood plain. Deposits commonly are thickest at 

 the boundary between the channel and the flood plain, because de- 

 crease in velocity of the water at such places favors the deposition of 

 sediment. This process results in the formation of natural levees. 



The. deposits at the mouths of large streams likewise are irregular. 

 Sand alternates with silt and clay almost indiscriminately both later- 

 ally and vertically. The sediments are poorly sorted and commonly 

 are mixed with plant fragments (R. J. Russell, 1936). Thick deposits 

 build up off the mouth of the stream unless longshore currents carry 

 the debris away. A factor in the development of longshore currents is 



