Introduction

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Abstract
The River Wharfe, Yorkshire, is practically unpolluted, and flows through mountain limestone, millstone grit and magnesian limestone. The observing and collecting stations at Grassington, Pool and Ulleskelf represent conditions at these 3 main stages of its course. Observations from June 1926 to Nov. 1927 showed suspended solids were usually less than 1 part per 100,000; nitrite occurred in measurable quantity only between June and Sept. at Pool, and between June and Oct. at Ulleskelf; ammoniacal N reached 0.008 p.p. 100,000 in April at Ulleskelf. A brownish coloration due to peat occurred in Aug. and Sept. each year, resulting in high O-absorption and organic nitrogen figures. The O content of the water never fell below 79.8% saturation. There was never any striking depletion of either phosphates, nitrates, or silica, as in lakes. The water was "slightly hard" (Thresh). The invertebrate fauna (except Protozoa, Nematoda and Rotifera) varied with the nature of the stream-bed and the rate of current. At Grassington and Pool the fauna was that of rapid water, but at Ulleskelf bivalves and gastropods predominated. In general, there was no marked seasonal change in the specific nature of the fauna, and variations in the character of fauna were largely related to length of life cycle, duration of period of maturity, and to current changes causing disturbances and translocation of organisms. The phytoplankton collections made from June to Sept. 1926 consisted mainly of vegetable debris and animal excrement, together with fragments of filamentous algae, and small diatoms. The flora appears to be mainly washings from stones and the river bed, and there were few real plankton spp. Few spp. occur all down the river; most were limited in distribution. Volvox aureus occurred at Ulleskelf in Aug. (Sept.), and Myxophyceae were more prominent here than elsewhere. At Pool, Rhizoclonium was abundant in June, and Spirogyra in July.

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