How to count upstream terminal ends of watershed network

[Kenai_Watershed_Forum_]

I would like to count the number of upstream terminal ends throughout a watershed. I.e., how many uppermost headwater polylines contribute to the river's outlet?

For example, for all streams in a single HUC that flow towards a single common outlet, I would like to know the quantity of origin lines that are connected to this outlet.

I am open to considering any approach or toolset that would help address this question.

I have attached an image that visually summarizes my question. The geographical area in question would contain thousands of points rather than just the five points illustrated here. The data source is the USGS National Hydrography Dataset.

Thank you in advance for your help. Using ArcPro 3.2.2 in Windows 11.
Visual summary of question

[SeaRM]

Wouldn't it be right if you split the lines with branches to the parts and after all just summarize them? Every line part has a start point and an end one.

[Kenai_Watershed_Forum_]

Thanks for the idea. However I am looking count only the number of most-upstream polylines, rather than all of the polylines in the network.

[MicZatorsky_AEC]

If you consider the terminals origins and the end of the drainage network as the destination, you can solve this with network traces, linear referencing, and Python. 

• Create a Trace Network from the streams
• Perform a network trace from each terminal to the destination and save that as a polyline.  You'll end up with a set of polylines.   
• For each polyline, turn it into a Route.  You'll have a set of Routes
• Locate each terminal along each route, set a very small neighbourhood distance.  This gets you the m-value of each terminal on each route.
• Merge the data to build a table of the Route_ID, Terminal_ID and the terminal m-value.
• For any given Route_ID, the most upstream terminal will have the lowest m-value

[Kenai_Watershed_Forum_]

Thank you, this sounds like the solution I am looking for. It is a bit more advanced than my current skill set, so it will be some time before I can execute. I would ultimately like to perform this analysis at a very large scale (watersheds covering around ~120 million hectares). I'll try some smaller examples first.

I'll let you know here how it turns out in a few months!