The Lower Thames Valley Conservation Authority is issuing a Low Water Level 1 Condition.
The move follows a similar alert for the upper portions of the river issued on Wednesday.
Officials say the area has received 20% less rainfall than average over the last three months.
River flows are less than 70% of normal for this time of year and if the region doesn’t see any rain, a Level 2, which means flows drop to 50% of normal, could be issued next month.