When Chiang Mai was first founded, it was too close to the Ping river. A few years later, it flooded, killing an astounding 100,000 people (this was in 1290 or something, so 100,000 people is a VERY large number.) They had the sense to move the city to its current location right afterwards. The old city and its temples was only rediscovered in the 70's or 80's.
This was a day we spent with a guide, who took us here and to the hills. He was very excited about buying and releasing some catfish for good karma. He was a great guide, and answered all of our questions knowledgably. It was great to learn so much about the city and culture.