California water managers said the late season rain threw them a curve ball this week.
The rain is coming down in the extreme northern portion of the state, but it is going to be felt as far south as Sacramento.
Heavy rain is expected late tonight and Friday in Redding and areas to the north.
The river may rise about a foot and a half in Sacramento in the coming days, leading to a warning for downstream boaters to expect especially fast, cold water and many floating logs this weekend.
Lake Shasta is 98 percent full, leading the U.S. Bureau of Reclamation to release water in anticipation of increased rain runoff and snowmelt.
Current releases are about 10,000 cubic feet per second, but managers are preparing for releases of up to 30,000 cubic feet per second. Last week and again on Tuesday, the Anderson Cottonwood Irrigation District lowered the level of its diversion dam near Market Street in Redding.
Irrigation district manager Stan Wangberg said that will prevent the dam from being bombarded and potentially damaged by floating debris if river levels are dramatically increased.
Managers of Shasta Dam said they still do not have any plans to allow water to flow over the top of the dam, which last happened in 1998.
However, if flows are increased above 18,000 cubic feet per second, some water will begin to flow out of valves on the face of the dam near the top.
KCRA.com contributed to this article.