44 



GEOMORPHOLOGY: 



fewer sediments for the stream to carry. However, 

 the stream still maintains much of its particle-carry- 

 ing capacity. Therefore, as the source of sediments 

 decreases at the headwater, the stream bed is cut 

 downward more and more by the main body of the 

 stream. During this stream bed cutting, the even and 

 slight slope characteristic of a mature stream is main- 

 tained; however, the lowering is indicated by the pres- 

 ence of steplike terraces (Figure 4.9). In these 

 terraces, each "step" was a previous level of the 

 mature stream that was stable for some time. The 

 different steps occur because the stream is slowly 

 but constantly changing its path. 



INITIAL 



TERRACES 



Figure 4.9 Development of stream terraces. A meandering mature 

 stream cuts through various layers of alluvium or other materials. 



The developing of meanders involves removing 

 materials which have further eflTects downstream. 

 At the stream's mouth, accumulating deposits plus 

 the local increase of meanders since youth contribute 

 to delta growth. As a stream ages, the delta increases 

 in size and becomes interlaced with branched and 

 interconnected meanders. By the stream's maturity, 

 the delta is well formed. 



To summarize, the most significant features of 

 maturity are emphasized by its two stages of develop- 

 ment. In the first stage, only the main stream has 

 constant slope and is mature; the tributaries are 

 young and have the features of young streams. The 

 second stage is reached when tributaries become 

 mature. Then, when many to all tributaries, and 

 even temporary stream beds, have constant slope, 

 old age is reached by a major part of the main stream. 

 However, the part of a main stream near the mouth 

 might become old while its headwaters are young. 



An old stream has very little slope. Because its 

 meanders become wider, its rate of flow decreases. 

 With a decrease in flow rate its sediment-carrying 

 capacity lessens, and it deposits materials upon its 

 own bed and edges. This causes the stream bed to 

 rise above the level of the surrounding flood plain. 

 The sides of the stream are contained by stream bed 



Figure 4.10 Old stream. Note the natural levees and lack of cutting 

 as in a moture stream. (After a Ward's Geomorphic model.) 



deposits in the form of natural levees (Figure 4.10). 

 Such a condition is often found near the mouths of 

 large streams and in the Mississippi River where 

 flood waters often break through natural (or artificial) 

 levees to cause great damage. 



If a peneplain or any other plain containing ma- 

 ture to old streams is elevated, the area is said to be 

 rejuvenated. Then the streams again start cutting 

 the landscape and a new stream life cycle is started. 



Siream Deposits and Delias. The site and rate of 

 deposition affect the kind of land forms produced. 

 The primary sites for streams to accumulate fine 

 particles, or alluvium, are on the flood plains, at the 

 mouth, in channels, in depressions, and at the base 

 of slopes of streams. These alluvia naturally accumu- 

 late in direct proportion to the deposition rate. The 

 rate is greater as the stream cross-sectional area in- 

 creases and both water volume and velocity decreases. 



Deltas are usually deposited wherever the velocity 

 of a river is checked by the mouth entering a large 

 body of water such as a lake or the ocean. Due to 

 decreased velocity of stream flow, alluvium accumu- 

 lates at the river's mouth. This deposition is graded; 

 first coarse materials and then finer materials are lost 

 by the stream. In an ocean situation, grading pro- 

 duces beaches at the mouth of the stream and sand 

 bars just off the shoreline. The bars further retard 

 stream flow and eventually further deposition and 

 stream development produce a delta of braided 

 streams, oxbows, and lakes all upon the flat, muddy 

 substrate of a salt marsh. These features are also 

 approached in lakes, especially the larger ones, but 

 there a fresh-water marsh form&r- 



