Hydrological characteristics and aquatic
environmental qualities
During the study periods, differences among daily discharges,
monthly average discharges and 3-month average
discharges of the Petch Dam were found (Fig. 2). Variation in
the discharges during November 2010 was apparent due to
the impacts of the flooding-induced high outflow rate during
October 2010. In contrast, the drainage volume in the natural
river was substantially reduced during May 2011 due to agricultural
need and, thus, the water was directed to irrigation
pathways. The discharge levels were increased again during
August and November 2011. The 3-month average discharge
finally reached 18.52 m3/s during the last survey period.
At the sampling sites,water temperature, conductivity and
pH ranged from 26.3–31.0°C, 95.0–167.0 μS/cm and 7.2–8.6,
respectively (Table 1).Water quality analysis reflected a good
water status, especially for the dissolved oxygen levels (ca.
7–10 mg/L). In contrast, the chlorophyll a concentration,
which reflected phytoplankton condition, was comparatively
low (0.67–4.01 μg/L). Such results are commonly found in
fast-flowing streams with sandy and/or gravel deposits (Biggs
1996). The primary oxygen producers are therefore the
aquatic plant life existing in the river ecosystem.
Sedimentary tool organic matter (TOM) varied among
three transect lines (Table 1). This implies an irregular flow
pattern in the area. Temporal study showed a decreasing
trend of TOM, particularly in the shallow zone during November
2010. In that period, strong flooding occurred; thus, the
fine substrate was removed, leaving only coarser particles.
Thereafter, organic matter gradually accumulated during the
low-loading period with comparatively lower flow rates (May
2011). During that period, the TOM content recovered and
reached 4.68%. The succession of aquatic plants in the
shallow zone was also found under such conditions (Figs 3
and 4).