indicate that the block formed at Sellwood Bridge about July 12 

 when the river discharge approximated 9^200 oofoS„ and the water 

 temperature 20°Co They likewise show that the dissolved oxygen 

 concentration remained below five parts per million throughout 

 the balance of the summer and momentarily rose to that concentra- 

 tion with a minor crest of 7j, 900 cofoSo passing the station Septem- 

 ber i9o The block lifted for the season about September 30 when 

 river flows rose above approximately 7j, 500 CofoSo and the water 

 temperature fell to approximately 17o5°Co 



The approximate picture of the formation and lifting of the 

 Tlillamette oxygen block during 1949 within the reach covered by 

 these studies can be tabulated as follovirs s 



Block Formed s s Block Lifted 



s- Approxo gApproxo gApproxo sgApproxo ? Approxo j Approx, 

 Location ? date ? River -* Water ss Date ; River ; Water 

 s ? Flow s Tempo s % % Flow s Tempo 



i S CofoSoJ C S8 S CofoSo 8 c 



One point illustrated by the data from the tributaries will be 

 of interest to both fishery interests and sanitary engineers » Apparently 

 none of the Willamette tributaries is a significant contributor to the 

 maia stem pollution burden in spite of the fact that certain of them-- 

 notably the Tualatin^ the Yamhill, and the South Santiam— -are known to 

 be seriously polluted from the work of Dimick and Merryfield and that 

 by Fish and Ruckero 



These data indicate thatj, at least under the conditions prevailing 

 during the summer of 1949, the tributaries are absorbing to an amazing 

 degree any oxygen-consisning pollution added to their upper reaches 

 before the water reaches the main stem. The Tualatin offers a good 

 example of the point in questiono Although pollution of the Tualatin 

 in sufficient intensity to kill fish was reported below the cities of 

 Hillsboro and Forest Grove during the summer of 1949, nevertheless the 

 dissolved oxygen concentration at the Athay Bridge sampling station 

 near the mouth remained consistently above saturation during most of 



14 



