Physical and Chemical Characteristics of the Hyporheic Zone of a Sonoran Desert Stream

The hyporheic zone of three reaches of Sycamore Creek, Arizona consisted of an average 63 cm depth of predominantly sand or fine gravel (0.5-5 mm). Sediments were highly porous (1923% interstitial space) and interstitial water volume was 3-4 times that of surface water. Spatial distribution of temperature, sediment organic matter, interstitial nutrients, and subsurface oxygen indicate that physical-chemical conditions vary greatly within the hyporheic zone. Much of the observed variability may be due to repeated disturbance by flash floods. Organic matter content of sediment was low (0.08% by weight), variable, and generally declined with depth in shallow portions of the hyporheic zone. Hyporheic water temperature was higher than surface temperature in regions beneath the wetted perimeter in summer. Nutrient concentrations of interstitial water were enriched compared to surface water; ammonium-N, SRP, and nitrate-N were 269%, 174%, and 327% of surface concentration, respectively. Sub-surface velocity was low (0.62 mm/s), but vertical exchanges were pronounced. Interstitial oxygen was high in regions of infiltration (downwelling), and was generally reduced in discharge regions (upwelling), but subsurface patterns were otherwise complex. Vertical linkages between surface and hyporheic zones provide a mechanism for mutual influences. Chief among these are replenishment of interstitial oxygen by downwelling (and enhancement of aerobic respiration), and nutrient enrichment of surface water at upwelling sites.