Organic Matter Dynamics in Sycamore Creek, a Desert Stream in Arizona, USA

1 Present address: Department of Biological Sciences, University of Nevada-Las Vegas, 4505 Maryland Parkway, Las Vegas, Nevada 89154-4004 USA. and Schumann 1968). Precipitation in the Sycamore Creek watershed averages only 34 and 58 cm/y in lower and upper elevations, respectively, and nearly all falls as rain (Thomsen and Schumann 1968). Evapotranspiration is quite high (pan evaporation = 310 cm/y); consequently mean annual discharge from this 5thorder river is only 0.8 m3/s (US Geological Survey 1989-1993), and on average only 8% of precipitation runs off (Grimm 1993). Rain at higher elevations in the watershed feeds permanent flow in lower-elevation reaches of Sycamore Creek. High-elevation precipitation recharges aquifers that drain into porous alluvium. Lower-elevation permanent reaches occur where geologic faulting brings impervious bedrock to the surface. Downstream from these "sources," surface flow may exist for a few m to several km before water seeps back into sediments. Rain is bimodally distributed with peaks during winter (January-March) and the summer "monsoon" of late July through September. Winter precipitation is usually gentle and spans one or more d, whereas summer rain occurs as short, localized thunderstorms. Runoff from storms produces spates (flash floods) in streams of the Sonoran Desert that are among the most severe in North America (Baker 1977, Ely et al. 1993). Winter storm flow typically remains elevated for some time. In contrast, summer thunderstorms frequently produce flash floods during which discharge rises rapidly then recedes within hours. Stream flow is lowest in summer