Streambed temperature patterns were measured in three riffles of a northern Michigan (USA) warmwater river using 95-cm long stainless steel temperature probes. In riffles with uniform small particle substrata, patterns suggested that streamwater infiltration occurred at the head of riffles, affecting substrate temperatures as much as 50 cm deep. At the downstream end of riffles, cool water was much nearer the streambed surface. Temperature patterns were very irregular when large rocks occurred in and on the bed. These data also suggest that streambed temperature patterns may be a useful tool in determining the presence and extent of hyporheic zones and in determining longitudinal and physical stream characteristics likely to indicate where hyporheic zones exist.