The Whitmer River is a river located in the northwest of Natalia, flowing out of West Wood and through Nick Hollow. It passes underground through Seven Mounds before winding its way down to Decker's Landing, a short distance from Cloud Mountain.
After discovering that the river flowed into and back out of Seven Mounds, Picket Longtreader and Smalden Joveson arranged to meet Wilfred Longtreader at the outlet. Picket and his sister Heather Longtreader discovered that the stream had been dammed at the outlet from seven mounds, but with Wilfred's help were able to break the dam. They then boarded a boat with Wilfred and the unconscious Smalls, and used the river to make their escape from Redeye Garlackson's wolves.
The group spent a day and a night on the river, intending to disembark at Decker's Landing only to find it aflame. They thus made their way to shore and left the river in order to travel to Cloud Mountain.