The river Mosel and its tributaries are home to Germany's oldest vineyards, dating back centuries to Roman times. A nice historical legacy is one thing, but it wouldn't mean much if the the region wasn't also the source of some of Germany's best-known wines--almost entirely Riesling packaged in tall green bottles. While the dominant environment here is steep slate slopes, even small variations in mineral content of the slate, subsoil, exposure, and other factors can make for quite dramatic and systematic variations in the resulting wines. Take, for instance, the top vineyards of neighboring Erden, Urzig, Zeltigen, and Wehlen, to the eye each just another towering wall of slate. Erden Rieslings are nearly always dominated by aromas and flavors of citrus, particularly lemon and tangerine. Rieslings from Urzig evince red berries--usually strawberry. And those from the top sites in Zeltingen and Wehlen are nearly always redolent of apple, vanilla, and nut oils.
Despite these differences, all of these vineyards benefit from a set of geographical factors that ensure consistently high-quality wines. The adjacent river helps to moderate temperature and reflect heat. Likewise, the slate soil helps retain heat, and the impressively steep angle of the slopes helps point the vines in the direction of the sun.
Besides the Upper and Middle Mosel areas--from Koblenz upstream to Trier--this large official growing region takes in the tributaries of Saar and Ruwer, which flow into the Mosel near Trier.
The Saar originates in the Vosges mountains, which separate Germany from Alsace, but its top vineyards only emerge along the final twenty miles of the river, before it flows into the Mosel. Here, the soils are less uniform and more stony than those downriver, and they retain less heat. Additionally, the Saar and her tributaries are smaller bodies of water than the Mosel, which limits their ability to serve as a moderating factor on the region's microclimate. As a result of these factors, Rieslings from the Saar often struggle to ripen, and have a reputation for being harvested even later than on the Middle and Upper Mosel.
The Ruwer River is even smaller than the Saar, all the way down to the point where it joins the Mosel, a few miles downstream from where the Saar enters. The two rivers do share similar challenges to viticulture, for again, unlike the Mosel, the Ruwer lacks the mass of water necessary to moderate temperature, and the surrounding areas lack the consistent layer of slate in the soil that allows the vineyards to retain heat evenly. Consequently, like the Saar, grapes from the Ruwer must also fight to achieve full ripeness. One easy to recognize result is that, in wines from both of these river basins, acidity generally outweighs richness.