I am pretty sure the gauge that has the uS/cm@25C is the one that you would want to look at to measure the clarity. It measures specific conductance, which is measuring the dissolved ions. It will rise and fall when the water is high/low, with all of the kicked up sediment. I believe the intention of these is to monitor mine runoff and other pollutants, I didn't read the entire link below. For the USGS, I think they are looking for those rises when it doesn't rain, etc. For the most part, it seems to follow the cft and height gauges but I'm sure there are times when it is low and stained and most likely reflects on this.
https://pubs.usgs.gov/tm/09/a6.3/tm9-a6_3.pdf