SUMMARY

Concentrations and loads of six nutrients and suspended sediment and rates of streamflow varied widely in the 14 sampled watersheds of the Lake Tahoe Basin. Greater amounts of precipitation generally fall on the western side of Lake Tahoe Basin and the streamflow runoff and nutrient and suspended-sediment concentrations and loads reflect that.

The highest mean annual runoff and instantaneous peak and measured streamflow for the 20 primary and secondary stream gaging stations were found at the Upper Truckee River at South Lake Tahoe, which has the largest drainage area. The highest unit runoff occurred at Blackwood Creek. The lowest runoff, instantaneous peak and measured streamflow, and unit runoff were all found at Logan House Creek, which has the smallest drainage area.

Water-quality data mostly fell within expected levels for the sampled stations in the LTIMP network, with the exception of elevated pH values for the Edgewood Creek at Lake Tahoe. Field pH values there ranged from 7.3 to 10.6 with a median of 8.8. Field pH's were higher on the east side of the basin, but consistently less than the median and maximum values observed at Edgewood Creek.

The highest concentrations for four of the six nutrients (dissolved nitrite plus nitrate nitrogen, dissolved ammonia nitrogen, total phosphorus, and biologically reactive iron) and suspended sediment, and second highest concentrations of 2 other nutrients (soluble reactive phosphorus and total ammonia and organic nitrogen) were found at a miscellaneous urban runoff station (Highway 50 Culvert to Edgewood Creek). The highest concentrations of other nutrients were found at another miscellaneous urban runoff station (Tributary to Edgewood Creek above clubhouse) for soluble reactive phosphorus and Third Creek near Crystal Bay (a primary station) for total ammonia and organic nitrogen. The higher total concentrations usually occurred during summer thunderstorm events and during rain-on-snow events in the winter/ spring periods. Lowest total constituent concentrations occurred during fall and winter periods, when streamflow was at its lowest. Dissolved constituent concentrations and specific conductance tend to increase in the low flow periods and tend to be lowest during prolonged snowmelt period. Concentrations of dissolved ammonia nitrogen were low for the study and had a large number of below detection limit values (less than 0.003 mg/L).

The emphasis in LTIMP sampling was placed on storm runoff and spring snowmelt runoff. As a result, samples collected under this study may have higher maximum concentration values, medians, and sums than from other water quality studies in the basin.

The Upper Truckee River watershed shows the largest monthly loads of all nutrients and suspended sediment of the 10 primary watersheds. The Upper Truckee River is the largest watershed and has the greatest annual runoff of the monitored watersheds in the Lake Tahoe Basin. Factors contributing to this high runoff are that the Upper Truckee watershed, located on the west side of the Lake Tahoe Basin, receives among the highest amounts of precipitation in the basin and has large amounts of developed area. The smallest loads for the 10 primary watersheds are from Logan House Creek, which is the smallest of the watersheds and has the least annual runoff for the 10 monitored watersheds. Logan House Creek generally is undeveloped (at least above the sampling station) watershed.

The highest monthly loads for the 10 primary watersheds generally occurred during snowmelt in June 1995 or January 1997, which included a major rain-on-snow event. The lowest monthly loads occurred during base-flow periods in August-September 1992, a drought year. Median monthly loads for the 10 primary watersheds also showed seasonality patterns, with most of the loading occurring during the spring snow-melt period (April, May, June). The smaller loads occurring during the summer (July, August, and September) and fall (October, November, and December) periods.

The largest median monthly yields occurred in the northeast part of the basin in the Incline Village area. Third Creek near Crystal Bay had the highest yields for total nitrogen, total phosphorus, biologically reactive iron and suspended sediment. Incline Creek near Crystal Bay had the highest yields for nitrite plus nitrate and soluble reactive phosphorus. Edgewood Creek at Stateline had the highest yield for ammonia. Logan House Creek had the lowest yields for all nutrients and suspended sediment. Of the 10 primary watersheds, the higher yields were from watersheds which receive greater precipitation and are more developed. The lowest yields were from two watersheds which receive less precipitation and are less developed.

The 10 primary watersheds were ranked based on nutrient and suspended-sediment yields. The rankings for the primary watersheds are listed from highest to lowest: Incline Creek, Third Creek, Blackwood Creek, Edgewood Creek, Upper Truckee River, Trout Creek, Ward Creek, General Creek, Glenbrook Creek, and Logan House Creek.

Constituent concentrations in the 10 primary watersheds showed decreasing or no significant trends, with the exception for biologically reactive iron and suspended-sediment concentrations. These two constituent concentrations decreased or were undetected at 7 of the 10 stations and increased at 3 stations. Decreasing trends for developed watersheds may be due to restoration projects or the introduction of best management practices. Causes of decreases in relatively undeveloped watersheds are unclear, but may be related to climatic variations. Increased trends of concentrations likely are influenced by samples collected during and after a large flood event in January 1997.