ASSESSING SPATIAL VARIABILITY OF NUTRIENT RUNOFF IN THE LITTLE CHAZY RIVER WATERSHED, NORTHEASTERN NEW YORK, USING MONTHLY SYNOPTIC WATER-QUALITY SURVEYS OVER A ONE-YEAR PERIOD

Management decisions regarding nonpoint source runoff to Lake Champlain are currently based on limited water-quality and stream-flow data. The available data, however, provide little or no information on upstream nutrient sources or sinks.

The Little Chazy River is a medium-size (basin area = 145 km², watershed length approx. 45 km) tributary to Lake Champlain that drains a predominantly agricultural area. This one-year study used a monthly synoptic water-quality sampling regime and stream-discharge data to assess the spatial and temporal variability of nutrient loadings (P, N, phosphate and nitrate) in the lower 34-km reach of the river. Eleven recording gaging stations provided stream discharge data along the main channel and at major tributary inflows within the study area. Each synoptic sample suite included 31 mainstream sites and 12 tributary inputs in the study reach and 20 additional sample sites throughout the remainder of the watershed.

Nutrient concentrations and loadings increase where the river transitions from forested upland to the Champlain Lowland. Superimposed on these general trends are small-scale variations that relate to spatially and temporally variable stream discharge, manure-application rates and distribution, small agricultural inputs such as drainage ditches or tile drains and physical or biological sequestration in small impoundments.

Nutrient loads also vary with flow regime. A single high flow (Q>10m³/s) event sample suite showed nutrient concentrations and loads increased steadily through the lowland agricultural areas. For example, the average rate of increase for total phosphorus and nitrate were about 4.5 and 240 kg/d/km, respectively, for this event. At moderate (Q=1-10 m³/s, N=7) and low (0.1-1 m³/s, N=4) flows, average total phosphorus and nitrate loads increased modestly, 0.1 to 0.2 and 1 to 15 kg/d/km, respectively. Downstream trends under lower flow regimes often fluctuated, primarily in response to local variability of stream discharge and nutrient sequestration. High-resolution nutrient loading studies such as this illustrate the complexity of river systems and may be useful in better understanding processes controlling fate and transport of nonpoint source pollutants.