Officials in central and southern Texas are worried about how much more rain will flow into streams and rivers after huge downpours on Sunday.
More than 5 inches of rain fell Sunday in Austin, and the National Weather Service reported 12 inches of rain in some spots.
The downpour forced organizers to cancel performances at the Austin City Limits music festival.
A flash flood watch was in effect overnight in much of central and west Texas, and forecasters predicted an additional 1 to 3 inches of rain Monday.
Only minor flooding was reported Sunday, but additional rain will flow rapidly over saturated ground and add to waterways still swelling from Sunday's runoff.
"My main concern is the weather of the next 72 hours, especially Tuesday night," said Michael C. Fisher, emergency management coordinator in Bastrop County, east of Austin. "We'll be at or just above flood stage on the Colorado River."
Heavy rain could bring flash floods, and "people still have the tendency to drive into high water," he said.
On the upside, the rain "may have broken a two-year drought on the Colorado," Fisher said.
In and around Austin, some creeks overflowed and nearly two dozen low-water crossings were closed, CNN affiliate KVUE-TV reported.
Police cruisers blocked traffic, and workers put up steel barricades.
Austin police rescued some people from vehicles stuck in water, but no injuries were reported, city spokeswoman Candice Cooper said.
A weather service statement said "a conveyor belt of moisture" from the Pacific Ocean was hitting moist air from the Gulf of Mexico to cause the heavy rain. It said the flash flood watch might need to be extended later Monday.