The Environment Agency has issued flood watches on the Rivers Uck, Ouse and Cuckmere after heavy rain yesterday and overnight.
Flooding is also possible along the stretch of water near Gatwick Sley and between Crawley and Horley.
The agency has warned motorists and homeowners in low-lying areas to be prepared.
A spokeswoman said: "Late yesterday afternoon the rivers started responding to the heavy rainfall.
"With the nasty autumn we have had, the ground water levels are still very high.
"If the situation was normal the water would drain into the ground. But because of all the flooding it has been running into the rivers."
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