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What a Difference a Day Makes: Spring Snowmelt in the Sierras
D. Peterson, M. Dettinger, D. Cayan, R. Smith, L. Riddle and N. Knowles
River discharge is a major control on the physics, chemistry, and biology of San Francisco Bay. Management of the Bay-Delta region often centers on river discharge issues including salinity penetration. Therefore, the more we know about the causes and consequences of the variations in Central Valley discharge (delta flow), the likely management actions are to cause inadvertent problems.

We know only the initial (pre-European settlement) hydrologic condition in much of the Sacramento-San Joaquin water shed via proxy methods. Peeling off the multi-layers of water management and land use effects to "see" the natural variability in discharge at high resolution is, perhaps, almost impossible. However, a major component of discharge is snowmelt, especially in spring. And the major processes controlling spring snowmelt are natural and at high elevations where the gaging stations lie above the fray.

In our opinion the crown jewel in gage sites for linking atmospheric circulation to discharge through the spring snowmelt signal is the Merced River at Happy Isles, Yosemite National Park (Fig. 1). Due to the foresight of early hydrologists, this station has continuous daily records from 1916 to present (Cobb and Biesecker, 1971; Lawrence, 1987).

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