In summer, a mild wind is often found on the shore of a calm river. This is accused due to

Since specific heat of water is more than the average specific heat of the material of shore of a river, warms up slowly in day time, but the shore warms up quickly in day time. As a result, air near the shore warms up quickly, its density decreases and the air rises up creating vacuum. Air from above the river blows to the shore to fill up the vacuum.

          In the night, river water cools down slowly and the shore cools down quickly. So air flows from shore to the river.