THE US ARMY CORPS OF Engineers (USACE) has repaired two scour holes in the downstream side of the 33-year-old Lower Monumental hydro dam on the Snake river. The job involved pumping more than 1529.2m3 of concrete into the two holes in the stilling basin 13.7m underwater, without dumping concrete into the river.

The scour holes are the result of decades of water spilling over the dam and eroding the concrete and bedrock. The two holes are each about 0.6ha in area and a maximum of about 4.3m in depth at places.

Divers had identified the extent of the damage in September and later prepared the site for concreting. The US$3.5M repair job included regular monitoring by a biologist who kept watch in a boat for fish that might be in harm’s way. Turbidity, oxygen content and alkalinity were measured.

The repair work was done in late fall so as not to interfere with fish migration. In addition to repairing the holes, USACE is also adding two spill deflectors to improve fish passage.