SUMMARY. I. IVriphylon chlorophyll u and ash free dry weight (AFDW) were monitored in nine rivers to examine the relative importance of flows and nutrients tor regulating periphyton biomass in gravel bed rivers.2. Mean annual flows in the rivers ranged from 0.94 to 169 m^ s"', mean dissolved reactive phophorus (DRP) from I..^ to hS //g I"', periphytie chlorophyll u from 4.6 to 73 mg m -, and AFDW from 2.8 to 16 g nr-.3. For eight ot the nine rivers NH^-N. DRP. total Kjeldahl nitrogen, total phosphorus and total suspended solids were correlated (P<().()1) with flow. and lor seven rivers conductivity was inversely correlated (/'<0. ()5) with flow.4. There was a hyperbolic relationship between flows and biomass, with chlorophyll a >1(H) mg m ' and AFDW >2() g m-occurring most frequently in flows of <2() m-' s'.5. Floods prevented the development of medium term (i.e. up to 2 months) maxima In biomass in five ofthe 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 59% 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 prc-flood biomass than the magnitude of the event.7. Biomass maxima were signiflcantly correlated (/*<().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 gravelCorrcspondcncc: Mr B.

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Abstract. The random field approach for gravel-bed roughness characterization, which is based on the presentation of bed elevations as a three-dimensional random field, is justified as an alternative to the characteristic particle size approach. We first show that the bed elevation distribution is close to Gaussian and then investigate gravel-bed roughness using the second-order structure function. The latter reveals two distinct regions: a scaling region at small spatial lags and a saturation region at large scales. The scaling exponent H (a form of Hurst exponent) appears to be isotropic and universal for both manually created "unworked" gravel beds (H = 0.5) and natural water-worked gravel beds (H -0.79). However, the gravel-bed roughness, in general, is not isotropic and should be characterized by three independent characteristic scales. A simple model of gravel-bed roughness based on the structure function parameterization is developed and compared with the characteristic particle size approach.

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