go to USGS
Go to the stream flow site and submit your question. The usgs folks respond quickly to questions.
Tell us what you learn.
To the best of my knowledge
– Last Updated: Sep-30-09 3:01 PM EST –
1. The USGS gages typically measure a flow depth with a float system or pressure transducer that is then associated with a calculated cross-section area for that particular flow depth. This is often called a rating curve. Using this along with an average channel gradient or slope in the vicinity of the gage allows for an estimation of discharge via formulas. The morpology of the channel bottom often changes with high flow events, so it is imperative that the cross-section be occasionally re-surveyed to verify it is up to date and valid. Gages are typically located where the channel morphology is going to be most stable, like around bedrock outcrops.
2. Sometimes they transmit via radio telemetry, other times they work through satellite uplink. Some older gages aren't even real time and they record data that has to be periodically downloaded/converted. Those naturally wouldn't show up on the web with real time data.
3. No idea, but they are fairly pricey I would imagine (beyond a typical private citizen's pocketbook). USGS won't just accept any data and publish it either.
Disclaimer: These are basic points above. USGS goes through a much more rigorous process to get their provisional data to you. But it should explain it a little better.
It was also interesting to find online images of the gauging stations. Some are quite large – at least larger than I expected.
I thought most stations were right at the bridges, but that doesn’t really seem like a great spot for them as debris and logs are often caught up on the bridge supports on small creeks.
The USGS site
has quite a write up that answers all your questions. How they are built, what they do, how they are calibrated, why they are needed. Good reading.