In order for an aquatic herbicide to function properly, it must remain in contact with the target vegetation for the length of time necessary for that plant to receive a lethal dose. Water exchange plays a huge role in the results one receives from the application of an aquatic herbicide.
Placing liquid herbicides in flowing water systems can be problematic. The treated water column is functionally moving away from the target weeds at some rate. The liquids injected into that water move with the current.
Knowing the required contact time is the first key step. The US Army Corps of Engineers has established contact exposure time requirements for a number of aquatic herbicides including the three products proposed for the Bonner County Program in 2008. Our proposal has a paper in the appendix that sites this data. There are two graphs the reviewers should give a look in this paper. The concentration exposure times for triclopyr and endothall are shown there. The curves generally show that more herbicide is needed when less contact time is expected and less is needed for longer contact times. These curves also show that in cases of short contact time, even when the maximum label rate is applied, it can result in no effect on the plants. For example, triclopyr must remain in contact with the target weeds for from 24 to 36 hours to be effective and with less than 6 hours contact time, no control will result. In a static water situation, maintaining these target concentrations may not be difficult. If the water is moving through the treatment plot as is the case in a river, the herbicide is leaving with the current at some rate. It is critcal to know what that rate is to plan treatments.
One way to truly understand what your water exchange is within a treatment area is to perform a dye study. The first picture in this article show one of our treatment vessels assisting the US Army Corps of Engineers treat portions of the Pend Oreille River with Rhotamine WT Dye. This dye is injected at a known concentration to a treatment plot. This dye floureces and can be measured. Using a field Fluorometer and a sampling design, you can measure the exact concentrations of this dye over time at different areas in your treatment plot. By sampling sites for a 24 hour period, you have the data necessary to show what the water exchange within that treatment site will be. Once you understand that, you can determine what you need to do to make your application work.
In areas with high water exchange that would move the herbicide away from the plants prior to them receiving a lethal dose; you can make decisions such as making staggered applications to keep the rates up, timing the application during lower flow periods, waiting until there are significant plant beds to hold the herbicide in place or determining that the site is not a candidate for a liquid application and moving to a granular.
Based on the work we have done over a number of years with this technology on the Pend Oreille River, and reviewing some of the treatment sites proposed for liquid applications it is probable that water exchange will have a major impact on efficacy if one time treatments are made in some of these sites.
As such we have proposed an optional Dye study at a few representative sites so the County will understand how this will impact the results you would achieve at these types of sites and help the County make decisions on how to best deploy these herbicides. 
