Aka Mizu-umi

Name
The Red Lake was named so after a battle near the Kawa sano Okami Jikan. The blood form the battle ran into the river and the river into the lake. Months after the lake was still red and as thick as blood. On the anniversary of the battle the water in the lake turns red again and blood washes up on the shore.

Major References

 * Legend of the Five Rings; Third Edition Page 305.