The Cascade Mountains create a rain shadow effect, blocking the wet conditions on the other side from the Pacific Ocean. The Columbia, Walla Walla, Snake and Yakima rivers provide much-needed irrigation, air circulation and cooling conditions. Ice Age floods left behind layers of sandy, volcanic soils which favor grape-growing and repel phylloxera (along with the cold winters). This means grapes are grown from single root stocks instead of the hybrids found in most of the world to prevent the spread of Phylloxera. Syrah, Merlot, Cabernet Sauvignon, Riesling and Chardonnay are the most popular varietals cultivated. These benefit from a long growing season that promotes proper phenolic balance in the grapes.