There have been two deaths at Lake Red Rock in two days. A 40-year-old woman was riding a knee board pulled by a boat last night on the Red Rock Reservoir, and she fell off. Authorities don’t know whether she hit her head on something when she fell, but attempts to revive her failed. She was wearing a life jacket. The woman’s identity is being withheld until family members are notified. Forty-seven-year-old William Nelson of Urbandale is presumed to have drown while swimming at the lake on Monday. Nelson was the 21st person to drown in Iowa this year. An autopsy is being performed on the woman who died last night. A state official who spoke with Radio Iowa this morning says her death could be ruled a drowning.