psmart
Active member
For small streams or short-term projects, stream flow can be measured by building a gauging weir across the stream. The flow-vs-head relationship of these devices has been well-know for over 100 years, so by measuring the head above the weir, you can calculate the flow rate. Of course, this only works where it's feasible to build a dam across the entire stream, and these structures tend to get damaged by ice and debris pretty quickly, so they're not used for long-term measurements.
Permanent, automated gauging stations measure the water level in the stream, and then convert this to a flow rate (often Cubic Feet per Seconds - CFS). This makes the stations fairly simple - all it has to do is measure water level, which can be done with a float or a nitrogen gas bubbler. Of course, any system must be well-protected from the stream flow, debris, ice, etc. There are also lots of manual gauges that just consist of a big ruler fastened to a suitable object, such as a bridge abutment.
To convert the gauge height to a flow rate, you use a stage-discharge curve, which is usually created by measuring flow velocity at multiple points across the stream. Combined with an accurate survey of the stream cross-section at the measurement location, this lets you calculate the total flow (CFS) at one time. The challenge is that you need to do these measurements at many different stages in order to create a useful and accurate stage-discharge curve. So these studies take a while to complete, and have to be conducted over multiple seasons and years to obtain good data.
The velocity at each point is measured with a calibrated windmill-type device that is inserted in the stream at each point. If the stream is shallow, this can be done by a person wading across the stream. For higher flows you would commonly use - a cable car. That's why you'll find them on a lot of streams. Not so much to help people cross the stream, but for stream gauging. (You can see a recent cable car and the associated gauging station on the Saco river in Conway, just upstream of the Route 302 crossing. It's a nice walk on the Conway rec trail.)
As for the Stillwater weir and cable car being at different locations, that's consistent with the speculation that they were both used for stream gauging. There would generally be no reason to build both at the same location. Instead, each was probably the best means of measuring flow at that location.
to avoid that area because, for me, it is cursed!!!! 
