Throughout the wine world, we find many noted growing regions that owe their distinction, at least in part, to the existence of river, or fluvial, terraces. This is not surprising, since many classic wine regions are situated along significant watercourses. Terraces often provide an excellent habitat for grapevines, combining extra elevation with effective drainage for both air and water. For the geologist, the study of fluvial geomorphology investigates the reasons why the landforms we observe have come into existence.
River Terraces in the Awatere Valley of New Zealand
Fluvial terraces are elongated, generally flat-lying areas that flank the sides of river valleys. They consist of a relatively level strip of land, called a “tread,” separated from an adjacent terrace of floodplain by steeper slopes called “risers.” Terraces are composed of older floodplain sediments and are quite variable. Lenses of sand, gravel and clay can be found throughout the soils. Since a terrace is set above the too-wet river bottom, vigorous plants like grapevines can send their roots deep into the subsurface to find adequate moisture from clay deposits in the substrate while benefiting from the elevated perch and natural drainage.
The Natural Equilibrium of a River System
Fluvial terraces are the remnants of earlier floodplains of a river system that existed at a time when the river was flowing at a higher elevation. A river system naturally seeks an equilibrium status. At its source, a river has a steep gradient that becomes gentler as it reaches its base level -- either when it flows into another river or into the sea. Without intervening forces, a river will ultimately achieve this profile. Reality, however, does not allow an undisturbed system. The land over which a river flows is not uniform, with relatively weak and resistant bedrocks that erode at different rates. A change in flow rate will upset the equilibrium. A river in flood stage has an entirely different dynamic than a quiet flow environment. Landmasses rise and subside over geologic time which also alters the balance of a river profile.
One of the more geologically recent worldwide influences on river systems has been glaciation. There have been at least four major advances of continental glaciers in the last two million years or so. When glaciers were at their maximum extent, much of the world’s water was locked in ice that was thousands of feet thick on the continents of the northern hemisphere. During these times, the sea level dropped 300-400 feet. This dramatically changed the equilibrium of river systems, allowing active channel cutting in the newly exposed land. When glacial retreats occurred, vast volumes of sediment poured out with the meltwaters, choking valleys with sediment burdens that rivers could not transport. With the last retreat of the great ice sheets, rivers have begun the process of incising new channels, leaving the old floodplains perched as terraces above the river level.
Gravel Mound in the Médoc of Bordeaux
Where in the world does this impact winegrowing? Perhaps the most dramatic example is in Bordeaux, where the famous gravel mounds of the Médoc are terraces deposited during recent interglacial periods. Some of the gravels have been submerged under the waters of the Gironde Estuary as sea level rose following the last glacial retreat, but the deep gravels of Pauillac, St. Julien, Margaux and Pessac-Léognan owe their existence to the river deposits of the last ice age.
St. Emilion and Pomerol, although further away from the Gironde, show similar terracing features. The famous estates of Petrus and Cheval-Blanc rest on 1.5 million year old river terraces of the river Isle. The diversity of soil types left in the wake of the glacial meltwaters include the gravel over clay of Cheval Blanc and the famous blue clay of Petrus.
River terraces are also very important in the villages of the southern Rhône Valley. A thick ice sheet covered the Alps during the Ice Age and the Rhône and its tributaries transported huge volumes of sediment during the melting periods. Châteauneuf-du-Pape is the most celebrated appellation in the southern Rhône and its vineyards lie on a series of terraces above the river. Other vineyards on the eastern side of the Rhône are often planted on the terraces that remain from the glacial outflow. Although the entire southern Rhône was impacted by the glacial outwash, the vineyards of Gigondas, Vacqueyras, Valréas and a number of other Côtes du Rhône Villages are on the river terraces.
Vineyards on Terraces in Valréas -- Southern Rhône Valley
River terraces are important as well in other parts of the Rhône Valley. Vines along the Drôme, a Rhône tributary and the source of Clairette de Die wines are planted on terraces above that river. The Grenache-based rosés of Tavel come from terraces on the west side of the Rhône. Where the Rhône meets the Mediterranean, we find as well that many vineyards of the Côstières de Nîmes rest upon terraces above the river.
River terraces and vineyards merge throughout the New World as well. The Wairau and Awatere Valleys that form the Marlborough appellation on the South Island of New Zealand offer many sites on terraces above those rivers. The four rivers that have carved the landscape of Hawke’s Bay on the North Island have left abundant terraces that are now home to noted vineyards.
The west coast of the US has experienced geologic uplift over the last several million years, giving rivers the energy to cut their valleys more deeply, leaving their old floodplains as terraces above the current river level. The Russian River and its tributaries in Sonoma County have many vineyards planted on old terraces. The Rogue and Applegate River valleys of southern Oregon exhibit the same phenomenon.
Vines on River Terraces in the Applegate Valley – Oregon
In other US places, vineyards in the Columbia Gorge, Yakima Valley and Horse Heaven Hills of Washington often benefit from their terraced environment. The gravelly nature of their soils makes the terraces along the shores of Lake Okanagan in British Columbia favored sites. The new Snake River Valley AVA in Idaho abounds in terraced locations for vineyards and, in the Grand Valley AVA in Colorado; vines are largely planted on terraces above the Colorado River.
Wherever one travels in the wine world, vineyards are closely associated with the rivers and streams that transect them. Look at the shape of the land the next time you are in wine country and think of the natural forces that created this unique site.