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- 7 - river station in the survey area once each cycle. All cycle II sediment samples were taken after the thundershowers in the area on August 22 and 23. Samples for bioassay were taken at each effluent stationdaily and field composited in such volume as to give 1 gallon composites at the end of a sampling period. The sampling of biological life was done primarily by means of a screen. The screen was copper, 36 x 42 inches, 18 wires per inch, supported with wood on the sides and a steel band along the bottom edge. Organisms were collected from an area of 1 square yard immediately upstream from the screen by picking up rocks and wiping them off in the water so the current would carry them onto the screen. After the screen was removed from the water, the organisms were picked from the screen, placed in glass vials, and preserved with formalin. They were identified, counted, and weighed at a later time. Processing As previously mentioned, bioassay samples were field composited prepara- tory to shipment. River and effluent samples were filtered through a 0.45~ pore diameter membrane filter within 2 days after collection and composited for ship- ment to the point of analysis at the end of each cycle. HYDROLOGY Stream flow data were obtained by two methods. At several of the sampling stations, staff gages, installed and rated by the U.S. Geological Survey, were read daily at the time of sample collection. On the Dolores River, gages were located so as to give flows corresponding to the stations above Burro Mine, below Slick Rock plant, above the confluence, and at Gateway. A gage was also located on the San Miguel River just above its mouth. The U.S. Geological Survey has a water stage recorder located on the San Miguel less than 1 mile above Uravan. Unpublished data from this recorder were furnished; these flows may be considered representative at the station just below Uravan as well as above town. Table III gives measured flows during the survey period. There was a steady general rain downstream from Slick Rock early on the morning of August 22 followed by a short intense rain in the vicinity of the confluence of the Dolores and San Miguel Rivers early in the afternoon of the 22nd. Early on the morning of the 23rd, there was a heavy thunderstorm in the Uravan area. On the 23rd, hourly flow data for the San Miguel River at Uravan recorded 35 cfs at the beginning of the day, 50 cfs at the end, with a peak of 622 cfs during the day as a result of heavy runo~f. There was not enough rain above Slick Rock to affect river flows there. Comparisons with published information indicate that the survey period was one of the driest on record. Published data for the two pertinent stations are summarized in Table IV; Figure II is a hydrograph for the San Miguel at Uravan for calendar years 1959 and 1960.