July is a good time to visit Emerald Lake, the largest of the lakes in Yoho National Park in the Canadian Rockies. That’s when the emerald green water is at its prettiest. The lake gets its color from the powdered limestone found in the water. The lake is generally frozen over until June, so waiting until July means the color will be at its peak. If you visit in the winter, however, it’s a great place to get in some cross-country skiing.