Several methods exist for the detection of nonlinearity in univariate time series. In the present work we consider riverflow time series to infer the dynamical characteristics of the rainfall-runoff transformation. It is shown that the non-Gaussian nature of the driving force (rainfall) can distort the results of such methods. in particular when surrogate data techniques are used. https://www.roneverhart.com/Pure-27-Blemish-Correcting-Purifying-Fluid/