We call them "rivers" and it's very common in the Sea of Cortez. We see it all the time when the water is flat, but usually not as distinct as your pictures close to shore. It has to do with current breaks and the tide change. The water on one side is usually just a slightly different temperature than the other side. We often troll the breaks like this when we find them in the summer. Further out, they often accumulate sargasso weed or appear as a scum line, like the white border that appears in your pictures.