Measuring drift during a receding flood: Results from an Austrian Mountain Brook (Ritrodat‐Lunz)

Abstract

AbstractThe drift of organisms and large particulate organic matter >200 μm (LPOM) was investigated during a single receding flood event from 16 to 23 June 1989 in a second order, calcareous, alpine, gravel brook. Starting with the peak level of the hydrograph, which was well above bankfull level, sampling lasted for five days (= 8 sampling dates). Between four and eight replicates were taken at each sampling date.No significant differences (P < 0.05) could be detected in the proportion of the main aquatic taxa (excluding miscellaneous taxa) drifting during above versus below bankfull water levels. However, terrestrial taxa were significantly (P < 0.05) over‐represented (23–25% of the total) at flood peak and a small secondary flood peak four days later. In addition, aquatic taxa which normally are scarce in drift samples at the Seebach (e.g. oligochaetes, ostracods) were abundant during the receding main flood event.Above bankfull stage (water level ⩾ 70 cm), animal drift densities were significantly (P < 0.01) and up to 22‐times higher (e.g. 45.6 individuals m−3) than during baseflow (e.g. 2.1 individuals m−3). Below bankfull stage, drift densities remain constant, independent of water discharge (Student‐Newman‐Keuls test; P < 0.01). In LPOM drift, this ratio was nearly 100: 1, with drift values ranging from 1.83 g dry weight m−3 at flood peak to 0.02 g dry weight m−3 at baseflow. Drift densities of animals and LPOM exhibited a positive exponential relationship with water level.Drift rates of anmimals and LPOM ranged from 3200700 individuals and 148.9 kg dry weight per hour at flood peak to 17440 individuals and 0.2 kg dry weight per hour at baseflow. During a single receding flood (water level ⩾ bankfull) significantly more organisms and LPOM were transported than during a whole year at baseflow discharge.