Hyporheic temperature patterns within riffles

This paper outlines the results of a pilot study using data at 12 min intervals from 20 miniature temperature dataloggers to establish vertical and longitudinal temperature patterns within a river bed. Data are presented from two adjacent riffles immediately below the Blithfield Reservoir on the River Blithe, Staffordshire, UK, for a five day period in July 1994. Hyporheic temperatures were warmer than those of groundwater, colder than those of surface water and decreased with depth into the river bed. At the heads of the riffles shallow sites (20 cm depth) mirrored surface temperature patterns but were on average 1.53 degrees C cooler while deeper sites (40 cm depth) were 2.60 degrees C cooler on average. Hyporheic temperatures lagged behind the surface water pattern and lag times increased with depth. Sites at the riffle tails generally displayed temperature patterns similar to those of the groundwater system: on average, temperatures were 4.32 degrees C cooler than surface water temperatures and showed no significant variation. Hyporheic temperature patterns at the heads of the riffles suggested downwelling surface water, while the tails appeared to be influenced by upwelling groundwater. Both riffles displayed similar hyporheic temperature patterns but riffle 1 was on average 1.22 degrees C warmer per site than riffle 2. Temperature differences between the riffles are attributed to bed form and substratum composition. These factors may have significant ecological implications including rates of organic matter decomposition, invertebrate life cycles and salmonid egg hatching times.