The entity that measures stream flow in the US is typically the United States Geological Survey (USGS). The method in which stream flow is measured is very labor intensive as shown here. Of course, in real time, this sort of time intense activity is impossible. Therefore, the USGS builds stage-discharge relationships and in real time a remote automatic gage measures the stage (height of water) and then the flowrate is looked-up from that stage-discharge relationship. This methodology is explained here.
There are several USGS stream gage sites in the area. Two of the very important ones are just above Lake Burton and the other is on the Chattooga just east of Clayton. The USGS nomenclature for these sites is Tallulah River Near Clayton, GA (02178400). and Chattooga River Near Clayton, GA (02177000). Note that the drainage basins of these two gage points are 58.4 square miles (Tallulah R) and 207 square miles (Chatooga R). Also, note that the ratio of these two drainage basins is 0.282 (58.4 divide by 207). Since these two flow points are about 10 miles apart and their drainage basins are adjacent to each other then they might have similar rainfall patterns and similar "nature of the land". Assuming they do have these similar features, the only other variable is the size of the drainage basin. Therefore, one might expect the flow at the Tallular R (02178400) is 0.282 times that of the Chattooga R (02177000). In fact, if you download the data from the USGS for these two sites for the time period 1965 through 2019 you will find the actual ratio is 0.277.
Estimating flow at one location based on known flow at another location when the rainfall and nature of the land is similar is a valid method of estimating, and is referred to as a "drainage basin rato" method. For additional information on estimating flow at one unknown site from know sites then these two sites might be of value: 1-general google and a 2-USGS Report.
A map showing all the USGS stream gages in Georgia.
