Where does one body of water end and the next begin?

[ScienceNostalgia101]

https://www.nhc.noaa.gov/refresh/graphics_at5+shtml/115127.shtml?cone#contents

 

So at this moment Hurricane Dorian is forecast to make a direct hit on Lake Okeechobee. If the floodwaters extend from the lake to the ocean, they'll essentially be the same body of water.

 

But then I remembered, lakes are connected to rivers, which are connected to the ocean. This got me thinking; where does one body of water end and the next begin?

 

Where does a river end and the ocean begin? Is there some threshold of salinity? If so, does that mean the boundary between a river and the ocean shifts as that threshold shifts?

 

How do we distinguish rivers from lakes/ponds? Is there some metric of the velocity of the water? Does increasing/decreasing flowrate therefore shift this boundary?

[Roamer]

umm, rivers flow, they start in mountains and flow to lakes, seas & oceans.

 

[CharonY]

I am not a hydrologists, but they have  a specific classification system for different types of boundaries where different water sources mix. Much of the classification seems to rely on water composition and source tracking, but I do not know about the specifics. It is not entirely arbitrary, though.