A 2-year study was made in the Redwood Creek and Mill Creek drainage basins of Redwood National Park to determine existing chemical water-quality conditions and to identify the effects of logging on water quality in the main stems and tributaries of the two basins.

Overall, the chemical water quality of the main stems and the tributaries is excellent, suitable for most beneficial uses. Dissolved-solids concentrations range from 25 milligrams per liter in the Redwood Creek basin and 21 milligrams per liter in the Mill Creek basin during the rainy season to 139 and 49 during the dry season. Water shifts from a mixed calcium-sodium bicarbonate-chloride type toward a calcium bicarbonate type from the end of the wet season to the end of the dry season. It shifts back toward a mixed calcium-sodium bicarbonate-chloride type from the end of the dry season to the end of the wet season. The pH shifts with the water type from a median value of 6.80 in the rainy season to 7.37 in the dry season. Nitrogen and phosphorus concentrations are generally too low to support nuisance algae but are high enough, in some streams, to support modest populations, particularly in the main stem where light levels are high. Trace-metal concentrations are low, typical of clean streams.

Evidence suggests that dissolved calcium and bicarbonate in stream water is produced by weathering of the Franciscan assemblage underlying the basins but that chlorides are transported inland from the ocean as dry fallout and spray and in rain. Exposure of the surface soils to the elements, either by logging or by natural causes such as sparse vegetation, seems to accelerate weathering, which leads to a calcium bicarbonate water type. Logging accelerates weathering most in the tributary watersheds with regoliths derived from sandstone and least in those with regoliths derived from schist; however, the data suggest that the rate of weathering in a schistose watershed can increase dramatically if soil disruption is extensive.

Studies during storms indicated that specific conductance and alkalinity were two to three times as likely to decrease at the discharge peak in logged watersheds as in forested ones. This suggests that overland flow containing lower concentrations of soil-derived dissolved solids than flow from other sources is a larger component of peak flow in logged watersheds than in forested watersheds.

Comparing a storm in November 1974 to one in February 1975, nitrate concentration increased significantly from November to February in a stream draining a logged watershed and decreased significantly in a stream draining a forested watershed. Then from the rainy season to the dry season, nitrate decreased in both logged and forested watersheds. This pattern suggests that soil nitrate produced by fixation and organic decomposition early in the rainy season tends to wash out of logged watersheds but be taken up in tree growth in forested watersheds. As the dry season progresses, base flow containing little nitrate enters the streams, causing a decrease in nitrate concentration. By contrast, the other plant nutrients--phosphorus, Kjeldahl nitrogen, ammonium, and dissolved organic carbon--all decreased in streams from the November 1974 storm to the February 1975 storm and changed little from the rainy season through the dry season. This pattern suggests that these materials tend to accumulate in the soil during the dry season and be washed out and diluted as the rainy season progresses. Very little reaches the water table due to soil absorption so that little appears in the base flow during the dry season.