River flow variability in Schoharie Creek, Catskills Mountains, New York, USA

Running water is an important agent that shapes the Earth's surface. Understanding changes in the volume of water moving through a watershed is one way to evaluate the power and work applied to the surface. Hydrographs, fancy word for a graph that shows the amount (volume) of water flowing past a point in a set time (seconds, hours, days, months) over a given time period (days, weeks, seasons, years), are the most common way to evaluate river flow (also called discharge). At present, agencies and researchers monitor flow in rivers around the world. In the United States, the United States Geological Survey (USGS) monitors flow in over 3000 streams and rivers (check out their webpage: http://waterwatch.usgs.gov) and in some instances have been monitoring discharge for over 30 years. What are the variables that impact flow through a watershed in a given year? Amount of rain? Amount of snow and rate of snowpack melting in the spring? Landuse changes? In this vignette, we are going to look at hydrographs from the Schoharie Creek station (Figure 1) over several time periods.

In hydrology, instead of following the calendar year, the water year is used as a standard timeframe. The water year starts October 1 and ends September 30 the following year, as in the northern hemisphere this follows from the beginning of the snow-fall through melt and spring runoff into the typically drier summers interrupted by rain events. For example, Oct. 1, 1939 – Sept. 30, 1940 is the entire 1940 water year (the year is assigned based on the calendar year that holds the most months of a given water year). A typical water year hydrograph for a northern hemisphere river shows lower flows through the winter, increasing through the spring and returning to low flows by the end of the summer (Figure 2).

At Schoharie Creek the data begins Oct. 1 1939 and continues to the present. If we plot the total runoff (volume of water) for each water year, starting in 1939, we can see what flow in Schoharie Creek was like over the last seven decades (Figure 3). It is important to note that flow in Schoharie Creek reflects low flows through the 1960s (regional drought) and higher flows in the 1970s (wet period in the region). For most of the record, high and low flows appear to be centered around 0.8 km³ per year. The notable exception is that the last ten years show a positive increase in flow, and some years are the highest flows on record.

As stated in the first paragraph, there are a number of variables that contribute to changes in flow (e.g., precipitation, timing of precipitation, landuse changes, water withdrawl). One way to investigate the timing of runoff events is to look at the annual hydrograph for different years. In Figure 2, the 1940 water year has a definitive start (around April 1), in which flows rapidly increase and remain high until it gradually recedes (flow decreases) as the snowpack is depleted. There is also a wet period in May and June where flow increases (but not as high as initial snow melt and spring runoff) and decreases. This pattern is typical for streams in the northern hemisphere. Over the decade 1940-49, this pattern persists (Figure 4); flows rapidly increase near the end of March as the snow melts and often increase toward the end of spring/beginning of summer. However in 2009 (Figure 5), and consistently during that decade (Figure 6), there are more flow events throughout the year, the volume of flow is greater and the timing of the highest flow occurs later in the spring. Throughout the winter, when flow was typically low, we see numerous runoff events. This can be interpreted as rainfall events, or rain on snow events, where the existing snowpack is melted by rainfall and contributes to large flow events. The shifts in timing and increase in flow have important implications for aquatic ecology and management of the river system itself.

The changes in the distribution of flow throughout the year in Schoharie Creek have significant implications on the residents of the valley and even people living further downstream (Mohawk Valley and Hudson Valley). Higher flows during typically low flow seasons will have implications for management and flood mitigation. Furthermore, the predictability in runoff will have major implications for people who depend on the water (e.g., recreation, agriculture). By evaluating the hydrographs at Schoharie Creek, we've been able to assess flow variability caused by runoff event timing and shifts in precipitation.

• Mohawk Watershed Symposium 2012, Abstract and Proceedings, URL: http://minerva.union.edu/garverj/mws/MWS_2012_Abstract_Volume.pdf
• Zembrzuski, T.J. Evans, M.L. 1989. Flood of April 4-5, 1987, in Southeastern New York State, with flood profiles of Schoharie Creek. U.S. Geological Survey, Water-resources Investigation Report 89-4084, 44pp.