Mission
Rivers are the least observed water we manage.
We can see a lake from a satellite and an ocean from a mooring. A river in spring is loud and opaque and won't hold still, and almost everything we believe about the fish in it comes from a few nights of sampling a year.
The gap
Decisions about rivers are made constantly and at scale. Dams are relicensed. Barriers come out. Seasons are set, recovery targets are written, and hundreds of millions of dollars of restoration go into channels on the strength of what is, in most cases, a very thin observational record.
The hard part is the measurement itself. A river you can't see into resists cameras and nets alike, and the fish that matter most are often the rarest ones in the channel (which is often why they matter). Sampling windows are short, and a lot of the work happens at night in cold water. The record stays thin, and the uncertainty in it ends up inside the decisions anyway.
What we are trying to change
Acoustics can watch a channel continuously, and has been able to for decades. What has kept it from becoming routine is that somebody then has to go through the recording. That work is slow and expert, and most of it is spent rejecting things that aren't fish.
Thalweg exists to make that reading cheap enough that continuous measurement becomes ordinary practice rather than a special project. A person still makes the call at the end. They just start from a season that has already been sorted.
Passage facilities and relicensing carry counting obligations that run for years, in water no camera can survey.
Take out a barrier and somebody has to answer whether the fish responded. That takes a season of measurement on each side of the removal.
Continuous coverage gives you run timing for the whole migration, including the nights nobody could be on the water.
Built to be checked
An instrument is only as good as your ability to check it. Everything Thalweg reports stays traceable back to the echoes it came from, so a count can be opened up and looked at.
Fisheries science already holds itself to that standard, and Thalweg was built by someone who has to defend the numbers in front of a committee.