21. The Oldbury Arm was cleared of debris and five weirs were removed from it to facilitate fish migration . 22. Thus the two dams combined became the first artificial barrier to anadromous fish migration up the Snake River. 23. In the 1930s, observers noticed the increasing pollution of the river, as fish migration from Laguna de Bay diminished. 24. They began incubating salmon eggs, clearing logjams and obstacles for fish migration and plant trees to sustain the riverbanks. 25. The dam and reservoir have fragmented the upper Green River, blocking fish migration and significantly impacting many native species. 26. Among them are that the cable must not interefere with fish migration and that it must create problems with corrosion. 27. To cope with fish migration problems, the dam features fish ladders to help native salmon and George Simpson in 1841. 28. In the 1920s, a private company built the Benbow Dam, blocking fish migration to a large area of the basin. 29. The dam blocks fish migration routes several miles below their natural end at the Great Falls of the Potomac River. 30. The bottom of the culvert will include baffles to retain materials which will create a low-flow channel to aid fish migration .
