17 

 Usinger: Introduction 





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Intro, fig. 20. Diagrams showing the formation of lakes by various geological processes. 

 a, a depressed fault block between two upheaved blocks; foreground the same, after a period 

 of erosion and deposition; b, great fault blocks of the northern Sierra Nevada, with the ploin of 

 Honey Lake in the east; c, coastal lagoon formed by wave-and-current-built beach; d, lakes formed 

 by volcanic action including crater and impoundments due to lava flows; e, glacial cirque lakes 

 with "hanging" troughs; f, river-made lakes including an isolated "ox-bow" and sloughs; g, lake 

 formed by a landslide (Davis, 1933). 



are low in productivity. In contrast to this the so- 

 called Eutrophic lakes are usually old, rich in nutri- 

 ents, and shallow because of the accumulation of 

 sediments and organic matter. The water in the photo- 

 synthetic zone exceeds that in the decomposition 

 zone (or may be entirely in the productive zone), 

 and biological productivity is high. Unfortunately, 

 the precise limits of these zones are difficult to 

 establish because light penetration varies with tur- 

 bidity, and photosynthesis is influenced by temperature 

 and many other factors. All that can be said is that 

 the Oligotrophic-Eutrophic classification is based on 

 the truism that autotrophic processes exceed heter- 

 trophic processes in surface waters whereas the 



reverse is true in deeper waters. With this as an 

 axiom, individual lakes can be classified as more or 

 less oligotrophic, and the process of aging or the 

 evolution of a lake into a pond and swamp can be 

 referred to as eutrophication. An alternative sequence 

 involves brown-water bog lakes and peat formation 

 with humic acids hindering organic decomposition. 

 These are called Dystrophic lakes. In California, 

 high elevation lakes are generally oligotrophic (their 

 beds having been scoured out of granite in recent 

 times by glaciers), whereas lakes of the lowlands are 

 at various stages in the process of eutrophication. 

 Dystrophic lakes occur infrequently and mostly at 

 higher elevations in the northern part of the state. 



