“Measurement of lengths of individual pools and riffles is partly a matter of judgment. With this qualification, we may say that we found in Seneca Creek that the average length of one repeating distance is 324 feet, which is 5.1 times the mean width of the bankfull channel. The ratio 5.1 in Seneca Creek compares favorably with the corresponding ratio, 5, for meanders in streams of comparable size. In Seneca Creek the average length of pool is 1.6 times the length of riffle. No corresponding figure is available for meanders.”