James L. Clayton

Mountain erosion over 10 yr, 10 k.y., and 10 m.y. time scales

We used cosmogenic 10 Be to measure erosion rates over 10 k.y. time scales at 32 Idaho mountain catchments, ranging from small experimental watersheds (0.2 km 2 )t o large river basins (35 000 km 2 ). These long-term sediment yields are, on average, 17 times higher than stream sediment fluxes measured over 10‐84 yr, but are consistent with 10 m.y. erosion rates measured by apatite fission tracks. Our results imply that conventional sediment-yield measurements—even those made over decades—can greatly underestimate long-term average rates of sediment delivery and thus overestimate the life spans of engineered reservoirs. Our observations also suggest that sediment delivery from mountainous terrain is extremely episodic, sporadically subjecting mountain stream ecosystems to extensive disturbance.

Salt Spray and Mineral Cycling in Two California Coastal Ecosystems

Windblown salt aerosols are a significant input to the mineral cycle along coastal areas. Cationic ratios of aerosol salts differ markedly from the same ratios in sea water. Resulting salt inputs to inland ecosystems reflect initial ion separation during sea—water bubble bursting and subsequent fallout of particulate salts until a base—level aerosol concentration is achieved. Inputs of aerosols to Baccharis brushland ecosystems are concentrated in plants and soil. Cations are trapped on leaf surfaces where they are available for foliar absorption and in some cases, translocation. Tracer studies with ² ²Na indicated that the greatest accumulation of retained ² ²Na was in the roots of Baccharis plants. Of the total applied dose of ² ²Na, 12% was either exuded or leached from roots of hydrophonically grown Baccharis over a 25—day period. This phenomenon may be considered a potential pathway for inputs of large amount of Na to soil near shorelines. The surface ionic content of soils reflects nutrient release from litter fall. At 1— to 2—m depths ionic losses from leaching occur in approximately the same ratios as inputs of salt spray. This suggests a steady state with regard to cycling of Na, K, Mg, and Ca.

Saturated hydraulic conductivities of granitic materials of the Idaho batholith

Saturated hydraulic conductivity (Ksat) of granitic bedrock in the Idaho batholith was determined using a borehole pressure testing technique. Tests were conducted at approximately 1.6 m depth increments ranging from about 1.6 m to an average maximum depth of 7.8 m. A total of 58 valid tests were obtained in nine holes located at five sites in a 145 km long transect line running north-south in the southwest quarter of the batholith. Sites represent a wide range in rock fracturing and weathering properties. Seismograph profiles were also run at each test hole. A conditional probability analysis showed that Ksat values were lognormally distributed with a lower bound at zero. Values for Ksat were unrelated to depth, rock matrix porosity, seismic velocity, or rock fracture density. However, Ksat did vary with rock weathering characteristics. Conductivity was lowest in unweathered rock probably because of restricted fracture apertures. The second least weathered rock class had the highest average Ksat. There was a general decrease in Ksat with increased weathering through the remaining five weathering classes even though rock porosity and fracture density increased with rock weathering. We believe this inverse trend in Ksat with increased weathering is the result of progressively increasing clay formation and mineral expansion that restricts flow in both fractures and the rock matrix. Ksat for bedrock averages an order of magnitude less than Ksat for soil cores and two orders of magnitude less than Ksat obtained by tracer tests during subsurface flow making shallow, sub-surface flow a major hydrologic process at many locations on the steep, mountain slopes.