LAND FORMS, THEIR DEFORMATION AND FORMATION 



45 



Drainage Patterns. Various stream, or drainage, 

 patterns are recognized (Figure 4.11). Dendritic 

 drainage systems have tributaries going in all direc- 

 tions in a pattern resembling the branching of a tree. 

 Rectangular drainage follows the rectangular surface 

 pattern of rock jointing which is sometimes produced 

 by weathering. Trellis drainage resembles rectangu- 

 lar, but the rectangular pattern of trellis stems from 

 straight tributaries to main streams. For this reason 

 onlv, trellis drainage displays many rectangles, each 



dendritic 



rectangular 



trellis 



radial 



Figure 4.11 Stream drainage patterns. 



having one side upon a continuous line. Trellis drain- 

 age is so named because it resembles the pattern of 

 vines on a trellis. More important, this drainage is 

 characteristic of landscapes having strongly folded or 

 dipped rock layers. Finally, radial streams, resem- 

 bling the radii from the center of a circle, drain an 

 elevation with a circular base, e.g., a dome mountain 

 or a volcano. 



Land Forms. Streams produce many land forms. 

 Only the commoner ones are summarized here. 

 Erosional forms include gullies, valleys, gorges, can- 

 yons, terraces, pot holes, plunging pools, and water- 



falls. Residual forms include mountain divides, 

 peaks, summits, natural bridges, and rocky valley 

 walls. Depositional forms include alluvial fans, flood 

 plains, and deltas. 



LAKES 



Lakes are usually closely associated with streams 

 and serve to regulate stream flow. This regulation is 

 important in preventing both flooding and drying of 

 streams. For example, when potential flood waters 

 enter a lake, the broad lake basin allows only a slight 

 rise above normal lake level and, therefore, only a 

 slight rise in streams exiting from the lake. On the 

 other hand, during drought or periods when little or 

 no water enters the lake, the broadness of the lake 

 again reduces the possible effect. Lake level is hardly 

 reduced and exit streams show little reduction in 

 normal water volume. 



The relationship between a lake and its streams 

 does not remove the possibility that a lake may show 

 marked fluctuations in height. However, whenever 

 there are great changes in lake water level, they must 

 come as a result of truly great changes in water 

 relationships. 



Life Cycle. Lakes are destroyed by processes that 

 drain and fill them. Drainage is accomplished by 

 evaporation and/or downcutting of exit streams. 

 Deposition comes from such things as delta building, 

 plant and animal remains, glacial deposits, and 

 shoreline cycles. 



Much of the shoreline cycle of lakes is like that 

 along the ocean. Erosion undercuts and removes 

 shore clifiTs, leaving a rock terrace. The eroded ma- 

 terial is further degraded to fine particles and the 

 particles are deposited as a wave-built terrace which 

 is a continuation of the rock terrace. Also, currents 

 within the lake may cause sediments to accurrfulate 

 either in offshore bars or in connections of land with 

 islands. 



Gradually, the process of deposition fills the lake. 

 As shallows form, certain areas assume the depth and 

 vegetation characteristic of swamps. Finally, further 

 deposition produces a transition from swamp to land 

 and, ultimately, the end of the lake's life cycle. 



PLAINS AND ELEVATIONS 



Discussion now turns from general destructive ac- 

 tivities of water to constructional or destructional 



