TRACER GRAVEL DISPERSION AND TRANSPORT IN A REGULATED RIVER

Water flow and sediment transport has been regulated for approximately 100 years below River Mill Dam on the Clackamas River in Northwest Oregon. Mitigation for documented downstream impacts include plans to augment sediment transport by adding 6,116 to 15,291 cubic meters of gravel annually to the river below River Mill Dam. On November 12 and 13, 2003 approximately 91 metric tons of gravel were placed in the Clackamas River at River Mile 22.3, below the dam. Approximately 15% of the gravel introduced was a unique tracer rock, a colorful quartzite, which does not occur in the Clackamas River above Barton (RM 13.5). Tracer rocks ranged in size from 9 to 75 mm in diameter. Gravel dispersed from the introduction site from an initial area of 260 m² to approximately 3,600 m², when observed in July, 2004. In August, 2010 10 bulk sediment samples, ranging from 68 to 145 kg were collected and examined to recover tracer gravel and determine the extent of tracer gravel dispersal. Clackamas River peak discharge averaged 762 cubic meters per second between 2003 and 2009, with bed mobilizing flows occurring every year after gravel introduction. Gravel has dispersed a minimum of 500 meters below the introduction site on both banks of the river. A large quantity of gravel was deposited on top of surface armor on a gravel bar located 450-500 meters downstream of the introduction site. Tracer gravel was also found in interstitial spaces between large surface armor near the introduction site (190 m downstream). Weight percent tracer gravel in bulk samples ranged from only a few tracer rocks to 0.57 weight percent. Recovered tracer gravel distribution and size will be combined with previously collected data and data from other regulated rivers to inform future augmentation planned by Portland General Electric (PGE).