In June 2006 the Quileute Tribe surveyed the lower section of the river (from Whitcomb-Diimell to the confluence with the Bogachiel). They recorded 447 sites. Almost half the sites had canes taller than six feet. 15% of sites had 200 or more canes; however, ">200" was the highest category available in the Data Dictionary at that time (more categories were added later). Some sites had many more the 200, possibly even thousands of canes. More than half of the sites recorded in the lower Sol Duc were identified as giant knotweed, in contrast to the mid-section, which was virtually all Bohemian.

In 2007 CCNWCB acquired six new Landowner Agreements for work in the lower portion of the river. A combined CCNWCB/Quileute crew treated all known sites on the lower Sol Duc. Rafts were used to facilitate access.

In 2008 the combined CCNWCB/Quileute crew monitored and re-treated all sites on the lower Sol Duc. Crew did not re-take data points there in 2008 because of time constraints and lack of satellites. A crew member made notes on maps which show that the largest site treated had 600 stems and on several sites the plants treated were under 3 feet in height, indicating that the first year's treatments were moderately successful in reducing the size and numbers of knotweed plants Crew found knotweed plants that had been treated in 2007 still flourishing on islands in the river near the Sol Duc-Bogachiel confluence and found plants in wooded areas of the floodplain which had been overlooked in 2007.

In 2009 the Clallam County Noxious Weed Control Board crew and the Quileutes spent 3 days in June treating the lower Sol Duc. All plants found were less than 5 feet tall and considerably less herbicide was used than in the previous year, but it is very likely that some re-growth occurred after the treatments were finished.

In 2010 the Quileute Nation's crew spent some time in late August surveying and re-treating the Sol Duc from Maxfield Road down to the confluence with the Bogachiel River. Previous treatments have been quite successful, but the Quileutes are still finding small healthy plants in places where there were none the previous year. In the fall of 2009 the Quileutes, while doing elk surveys, had spotted a couple of large patches in an overflow channel of the Sol Duc, and were able to treat them in 2010.

It will be seen that herbicide use increased in 2010, probably because plants were treated later in the year, more were found and they were bigger and took more herbicide to treat.

All known knotweed infestations in the Sol Duc River from the Olympic National Park boundary to the Sol Duc's confluence with the Bogachiel have now been treated for four years. A considerable decline in bio-mass has been observed and many sites have no visible knotweed plants.