14 



Usinger: Introduction 



Speed of current has been used as a criterion in 

 classification. Speed, like size, is subject to seasonal 

 fluctuation and also to diurnal fluctuation in snow- 

 melt streams. Readings are not always reliable because 

 of local variations but, in general, as pointed out by 

 Huet (1949), velocity is greatest a little below the 

 surface near the middle of a stream. Also, a stream half 

 as wide and deep as another, given the same slope, 

 wilL have a slower velocity (intro. fig. 16). Ricker 

 (1934) classified streams arbitrarily as "slow" — less 

 than 1.5 feet per second, and "fast" — more than 1.5 

 feet per second. 



Type of bottom. — There is a positive correlation 

 between speed of current and type of bottom. Tansley 

 worked out the details for streams in the British 

 Isles (quoted by Macan and Worthington, 1951) as shown 

 in table 1. 



Table 1 (Introduction) 



Relation of Current Speed 

 and Nature of River Bed 



Velocity of current 



Nature of bed 



Habitat 



per second 







More than 4 ft. (1.21 m.) 



rock 



torrential 



More than 3 ft. (0.91 m.) 



heavy shingle 



torrential 



More than 2 ft. (0.60 m.) 



light shingle 



nonsilted 



More than 1 ft. (0.30 m.) 



gravel 



partly silted 



More than 8 in. (0.20 m.) 



sand 



partly silted 



More than 5 in. (0.12 m.) 



silt 



silted 



Less than 5 in. (12 cm.) 



mud 



pondlike 



Ricker (1934) recognized several types of bottom 

 materials including bed rock, boulders, stones, gravel, 

 sand, mud, and plant debris. Each of these types 

 occurs in California and, in general, they are corre- 

 lated with speed of current and hence also with slope, 

 size, and so on. Typically the smallest streams of 

 Sierran and north coast forests have bottoms of gravel 

 and stones with much plant debris; the high mountain 

 streams of great velocity have scoured down to bed 

 rock; and rivers of intermediate elevations have 

 bottoms of boulders which give way in turn to gravel 

 and sand in the foothills and sand and mud in the 

 valleys. 



Temperature. — Temperature has a profound effect 

 on aquatic organisms, both as a direct factor influ- 

 encing physiological processes and as a limiting 

 factor on dissolved oxygen. Ricker (1934) settled on 

 75° F as the maximum temperature that separates 

 "warm rivers" from "trout streams." Actually, opti- 

 mum figures may be more meaningful — 66° F being 

 optimum for cold trout waters and 85° F optimum for 

 warm bass waters (Needham, in litt.). In California 

 the cold-water-warm-water line dips deeply below the 

 generally cool Sierran Zone (intro. fig. 13) or boreal 

 evergreen coniferous forest. Actually, although varying 

 with the season, trout extend down to the lower part 

 of the Californian or Upper Sonoran Zone on each 

 side of the Sierra, and in the northern Coast Range 

 and southern California mountains where vegetation 

 becomes sparse, slopes are gradual, current is slower, 

 and the less protected waters rise in temperature. 



Productivity. — Theoretically, productivity should 



be the best basis for stream classification because 

 it is determined by the interaction of all factors. 

 However, sampling methods are not adequate, and 

 faunal differences would confuse the picture in a 

 large and diverse area like California. In spite of 

 these and other difficulties, productivity has been 

 used in a general way for classifying streams. Patrick 

 (verbal communication), for example, speaks pf oligo- 

 trophic (poor in nutrients) streams in head-water 

 regions. These are clear and hence entirely exposed 

 to the sun's rays for photosynthesis. They are high 

 in dissolved oxygen, low in temperature, poor in 

 nutrients, and relatively low in productivity. At 

 intermediate elevations temperature rises, velocity 

 decreases, and productivity is greater. Then, rivers 

 pass through the foothills and out onto the alluvial 

 plains, picking up silt and then dropping it onto sand 

 or mud bottoms as velocity decreases. The muddy 

 waters block photosynthesis except near the surface. 

 By this time, algae and other plankton organisms 

 have increased so that the water can be considered 

 eutrophic (rich) but the bottom fauna is very restricted, 

 consisting mainly of Chironomid larvae. 



Productivity in terms of bottom fauna has been 

 studied extensively. In general it has been found 

 that riffles are more productive than pools, that rubble 

 and gravel bottoms are more productive than bedrock 

 and sand in that order (Pennak and Van Gerpen, 1947), 

 and that plant beds in streams are very productive 

 (Needham, 1938). Productivity was used by Hazzard 

 (1938) to classify streams. His standards were: Grade 

 I (rich stream) — more than 22 grams of bottom organ- 

 isms per square meter or 2,152 organisms; Grade II 

 (average) — 11-22 grams or 1,076 to 2,152 organisms; 

 Grade III (poor) — less than 11 grams or less than 

 1,076 organisms. Such figures have been shown 

 (Needham and Usinger, 1955) to be of doubtful signif- 

 icance statistically because of shortcomings of the 

 sampling equipment and techniques. Nevertheless 

 they give an idea of the general levels of productivity 

 and hence are of some value. 



Figures for California streams have been given by 

 Needham (1934, 1938, 1939). The maximum was 4,400 

 organisms in a single square-foot sample taken from 

 the Klamath River near Hornbrook, California. The 

 total wet weight of this sample was more than 105 

 grams. Seasonal variation was shown by samples taken 

 in Waddell Creek near Santa Cruz. Riffle-dwelling 

 forms varied from a low of 70 pounds per acre (extra- 

 polated from representative square-foot samples) in 

 February to a maximum of 472 pounds in May, 1933. 

 Comparable figures for the Merced River in Yosemite 

 National Park were 103 pounds in February and 85 

 pounds in August. 



All these figures apply only to gross wet weights 

 and total numbers at particular seasons. True produc- 

 tivity should be based on dry weights (and perhaps 

 on nutritive values as fish food) and should be sampled 

 and calculated as a standing crop or as an annual 

 crop, taking into account the emergence of successive 

 generations. Unfortunately, such an ideal study has 

 never been made, but with improved techniques and 

 increased knowledge of the biology of aquatic insects 



