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Notes: SUMMARY. 1. Periphyton chlorophyll a and ash free dry weight (AFDW) were monitored in nine rivers to examine the relative importance of flows and nutrients for regulating periphyton biomass in gravel bed rivers.2. Mean annual flows in the rivers ranged from 0.94 to 169 m3 s−1, mean dissolved reactive phophorus (DRP) from 1.3 to 68 μ g 1−1, periphytic chlorophyll a from 4.6 to 73 mg m −2. and AFDW from 2.8 to 16 g m−2.3. For eight of the nine rivers NH4-N. DRP, total Kjeldahl nitrogen, total phosphorus and total suspended solids were correlated (P〈0.01) with flow, and for seven rivers conductivity was inversely correlated (P〈0.05) with flow.4. There was a hyperbolic relationship between flows and biomass, with chlorophyll a 〉100 mg m −2 and AFDW 〉20 g m−2 occurring most frequently in flows of 〈20 m3 s−1.5. Floods prevented the development of medium term (i.e. up to 2 months) maxima in biomass in five of the rivers, but maxima occurred over summer-autumn and winter-spring in the three rivers where floods were absent.6. Chlorophyll a biomass was more resistant to flooding than AFDW. Only 5993 of the forty-six recorded floods caused chlorophyll a scouring, whereas 74% of the floods caused AFDW scouring. The efficiency of scour was more influenced by the pre-flood biomass than the magnitude of the event.7. Biomass maxima were significantly correlated (P〈0.01) with mean DRP concentration during the accrual period. Overall, up to 53% of the mean annual biomass difference between rivers was explained by the mean annual DRP concentrations. However, the high correlations between nutrient concentrations and flow indicated that the nutrient data were also carrying hydrological information and that simple causal relationships between nutrients and biomass are difficult to establish in rivers.8. It is concluded that hydrological factors contribute at least equally with nutrients to the differences in periphyton biomass between the gravel-bed study rivers. They combined to explain up to 63.3% of the variance in biomass, compared with 57.6% for nutrients. It is recommended that periphyton data from gravel-bed rivers should always be viewed within the context of the flow history of the site, and not just as a function of nutrient concentrations.