The ubiquitous dissolution of soils and sediments results in dissolved loads of major ions and nutrients to streams. In most temperate watersheds, the rate of weathering is generally positively correlated to temperature and precipitation. In the cold and dry McMurdo Dry Valleys of Antarctica, dilute glacial melt water generated during 8-10 weeks in the austral summer flows into well-defined stream channels, which spill into closed basin lakes. The sediments of these streambeds are unconsolidated and relatively 'fresh' (i.e., minimally weathered), underlain by permafrost at depths of ~75 cm. Stream water actively exchanges between the open channel and the thawed streambed sediments, known as the hyporheic zone ("hypo-" from the Greek meaning 'under' and "rheos" meaning 'river'). When thawed, water stored in the streambed actively dissolves the minerals there. Thus, as the dilute stream water exchanges with the high-solute hyporheic water and stream water, concentrations of silica, potassium, etc. increase downstream. Watershed areas of dry valley streams are small because there is no rain. Hence, chemical weathering yields per unit area watershed are some of the highest on earth.