In this paper we address an observational validation of recent theoretical results on the structure of the probability density function (pdf) of daily streamflows through the analysis of data pertaining to several catchments covering various sizes, climatic regimes, and topographic features across the United States. Seasonal streamflow pdfs obtained from recorded time series are directly compared with the theoretical distribution derived by Botter et al. (2007a) by coupling a suitable transport model with a stochastic description of runoff production through soil moisture dynamics. The ecohydrological and morphological parameters defining the theoretical streamflow pdf are inferred for each watershed on the basis of easily gathered land use information and incorporate directly measured hydrologic and climatic data. An excellent agreement is shown with the corresponding observed distribution in variably sized and widely heterogeneous catchments across different climate regimes. In particular, our data confirm that the shape of the daily streamflow pdf shows different regimes well described by a Gamma distribution. Such regimes, roughly termed wet and dry, are controlled by the ratio between the runoff frequency and the inverse of the mean residence time of subsurface flow, which in turn controls the behavior of the basins for low-stage streamflows.