What is the name for a submerged log or tree in a river?

Deadhead = a sunken or partially sunken log.

http://www.dictionary.com/browse/deadhead

A snag:

a tree or branch embedded in a lake or stream bed and constituting a hazard to navigation

https://www.merriam-webster.com/dictionary/snag

With submerged timber - the logs are called "sinkers."
http://www.nytimes.com/2005/04/21/garden/21wood.html

Big submerged logs (called SAWYERS) could puncture a boat and sink it.
mdc.mo.gov/kids/out-in/2003/01/2.htm webpage no longer exists

Deadheads (also called sinker logs or snags) – a submerged log causing a serious hazard. Impact at a high speed with a deadhead can punch a hole in a large boat or destroy a small one.
http://presentationrentals.blogspot.com/2007/04/boating-slang-glossary.html

Also, another reference for sawyer:
https://books.google.com/books?id=lxge9bv1Z-wC&pg=PA202&lpg=PA202#v=onepage&q&f=false

Probably known as a sawyer because it saws a hole in your boat. SLC got his name from the essential job of swinging the lead on the river boats. Two fathoms was called out as 'By the mark twain'. There would be distinctions for near the mark, but probably not the need for tallow in the end of the lead to test the surface of the bottom (for want of a better term) that were needed in saltwater navigation. Four fathoms was Mark Four, but I can't find any others. Very good thing here about riverboats and if it doesn't load first time, try again.

[bigeasy.com/features/captain.html, no longer online]