The majority of water in northern Thailand is supplied
by four main rivers (Ping,Wang, Yom and Nan).
The River Nan originates in the Luang Prabang mountain
ranges in the north-east of the northern area, and
stretches over about 740 kilometers passing through
the Sirikit Dam and some major cities. It flows southward
and meets the Rivers Yom and Ping at Phijit
and Nakhon Sawan provinces, respectively (Figure 1).
During the south-west and north-east monsoon period
(May to November), the high precipitation usually
causes temporarily flooding of vast areas. The samples
for this study were collected from 11 habitats, ranging
from paddy fields, ponds and canals to reservoirs.