We measured winter thermal characteristics longitudinally in four third-order streams, two each in the Idaho Batholith and Blue Mountain ecoregions. From November to April we measured temperature along 3.7 km to 11.9 km of stream at six to eight sites. Streams were snow covered for most of the winter. There was a seasonal pattern of stream-wide cooling in November and warming in April at all streams. In the mid-winter months (December, January, and February), the sites in the Blue Mountain streams had mean monthly maxima ranging from 0.2 to 3.0 °C and minima -0.1 to 1.5 °C stream-wide, whereas both Idaho Batholith streams were near 0.0 °C throughout their length and had monthly ranges of 0.2 °C or less. Winter temperature differences between streams of the two ecoregions create substantial differences in cumulative degree-days which could be a factor in the structure of instream biotic communities.