The most common and most likely answer to this crossword clue is the 4 letter word SILT
I've seen this crossword clue in Independent
S
I
L
T
Buddy explains!

The straight part
We know that "river deposit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case river deposit = silt.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SILT mean?
SILT noun- Mud or fine earth deposited from running or standing water.
- (by extension) Material with similar physical characteristics, whatever its origins or transport.
- A particle from 3.9 to 62.5 microns in diameter, following the Wentworth scale.
- To clog or fill with silt.
- To become clogged with silt.
- To flow through crevices; to percolate.