Climate can induce significant regional water-quality variations. Warmer areas undergo more rapid and complete weathering, generating large quantities of cation-depleted clays. Easily transported by running water, abundant clays contribute to the load of suspended particulate matter (SPM) and can carry contaminants. Differences in mean annual temperature thus explain indirectly (through the connection between temperature and evapotranspiration) much of the variation in extent of soil weathering and in the amount of suspended load in East Coast rivers, where SPM tends to increase as one moves south. The arid climate of the West meanwhile, enhances evapotranspiration, which can concentrate dissolved and suspended matter carried by streams and rivers.

Differences in bedrock cause most of the evident differences in regional water composition. Carbonates weather much more extensively and rapidly than silicates. Watersheds underlain mainly by silicate bedrock tend to have rivers with low ANC, pH, hardness, and total dissolved solids (TDS), and are dominated by the anions chloride and sulfate. In areas where carbonate rocks are common, hard water with high TDS occurs, and the most common anion is bicarbonate. The larger a watershed, the more likely it is to contain areas of carbonate bedrock. Thus large rivers tend to have chemistry reflecting carbonates dominating over silicates.

If evaporites (halite, gypsum) occur in a watershed, water may have high concentrations of chloride or sulfate, but pH is unaffected. Because of their very high solubility, evaporites are less common in humid regions, so they are more abundant in the West, but they underlie broad regions of the country. Finally, if reduced minerals are plentiful, their oxidation releases protons that cause stream acidification. An extreme example is found in the vicinity of mining operations, where coal and metal ores are found in association with metal sulfides. Streams in these areas can have pH as low as 3.

Land use and land cover, which influence water quality, also tend to have regional patterns. Many of these represent human-induced perturbations (e.g., agriculture and urbanization) as discussed later in this chapter. Populous regions and those where agriculture is extensive are likely to have degraded water quality. Another land cover, natural wetlands, covers a large portion of the landscape in several states (e.g., Minnesota, Maine, Florida). Waters from wetlands tend to have high levels of dissolved organic matter (humic and fulvic acids). Arid regions lack vegetation, which retains particulate matter, the combination of low vegetative cover and high evapotranspiration tend to cause high concentrations of SPM in western rivers.

Wide variations in the chemical composition of water are a natural consequence of regional differences in soils, bedrock, atmospheric inputs, and climate. In each region, aquatic ecosystems have adapted to local chemical conditions. Measurements of water quality in undeveloped headwater streams provide local benchmarks against which watershed managers can compare water quality degradations. Differences between ambient water quality and the local baseline are

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