The temperature of a small stony stream

Abstract

1. Records were obtained with a thermograph over a period of five years in a small stream. 2. Temperature fluctuated from 0°C to above 6°C in January and February; rose during the period from just before the equinox to just after the equinox with daily fluctuations up to about 6°C; remained steadily. Large daily fluctuations were rare in the second half of the year. The highest temperatures was 19.0°C. 3. The highest temperatures were reached when the sun shone strongly after rain, not in dry spells when the water level was low. 4. Temperature fell in a man-made underground channel. In another stream it dropped from 21.6°C out in the open to 14.0°C in a wood facing east on a hot day when the wind was blowing from the east. 5. Temperature reached a minimum at about 6 a.m. and a maximum nearly 12 hours later. 6. Temperatures lower down the stream, fluctuated more; the difference between the maxima was greater than that between the minima. 7. From the present and from published observations it is concluded that small streams warm rapidly and reach equilibrium a few km from the source, the average water temperature being not greatly different from the average air temperature. 8. A given number of degree hours is reached in spring three to four weeks earlier after a warm winter than after a cold one. 9. Average weekly temperature obtained with a thermograph and with a maximum and minimum thermometer read once a day do not differ greatly. Even a weekly reading of the latter gives a fair picture over a long period.