Typically the thalweg is the deepest part of the channel and the thread is the split between the banks. It does matter since the US changed the line to at least both of those, over the years, for the Rio Grande river as an international border. That said, for most riparian rules, they're pretty close. Rarely have I had to map both sides of a bank to determine the center if I can get down in it. Then it's probably irrelevant for my area of Texas where I'm trying to locate a gradient boundary which is no where near either of them.
Here's a thalweg, fun read:
https://scholar.google.com/scholar_case?case=5084538364819674265&q=thalweg&hl=en&as_sdt=4,33
I particularly like this part that indicates they found an engineer that likes kayaking:)
"and had extensive RECREATIONAL and professional experience in "reading" the effect an obstruction will have upon the flow of a river or stream"
How far from the thalweg to the Mark Twain?
The only time a thalweg should be deemed important is in sailing in or out of a harbor. In which case the channel (never in all my days sailing would we ever call it a thalweg) is marked by buoys.
Maintaining the red buoys on the right when entering a harbor ensures you will be in the channel (thalweg). Hence the sailor phrase "red right returning" was born. I am sure no surveyors set the buoys to mark the channel (thalweg).
Truth. I've needed to find the thalweg for bridge scours and thread for boundaries.
Im sad I am so late getting to this thread. Like others said, the answer on the FS is deepest part of the channel. I argued on Reddit a while back about it: https://www.reddit.com/r/Surveying/comments/17e2obr/survey_quiz_made_daily_not_posted_to_reddit_daily/&ved=2ahUKEwjz_NTn-JiHAxXnKkQIHXmJA_4QFnoECDQQAQ&usg=AOvVaw3lYR_hBWSHwkV4ZS6WkfQ 6">Survey Quiz (Made daily, not posted to ...Reddit · r/Surveying40+ comments · 8 months ago
The word has changed meaning over the years, as can be seen in its change from Black's Law Dictionary Ed. 1 to Ed. 9 (or whatever number they are on now)... it used to be the navigable line and not the deepest, but now it is generally seen as the deepest. The way I remember is that the thread of a stream is a boundary that might've been made so a farmer's cattle would be able to access running water in summer when a stream is low, and a thalweg is a much larger body of water at the navigable channel which is usually the deepest part...
What no one has talked about yet (I think) is how almost every person you meet mispronounces this word as THAL-WEG and not TAL-VEG.
