Tributary effects in rivers: interactions of spatial scale, network structure, and landscape characteristics

Landscape characteristics in combination with the physical structure of branched stream networks define the environmental conditions available for lotic biota. From simple stream network “laws” can emerge a spatially explicit understanding of habitat heterogeneity. Based on geographic information system analyses, we explore how stream networks integrate spatial heterogeneity of the landscape and form new characteristics as stream segments accumulate into progressively larger drainages and how these changes in landscape characteristics relate to confluence symmetry ratio and drainage size. Simple expectations for stream networks include the following: (i) abrupt changes in longitudinal patterns are more probable among the numerous small and diverse headwater streams than in large, rare, and characteristically similar tributaries, (ii) the many small tributaries flowing into large mainstem channels cause individually small, yet collectively gradual, changes in longitudinal patterns. Such a spatial understanding of where change is likely to occur helps to reconcile gradual river continuum and abrupt discontinuum views of patterns in rivers and predict the locations of important confluences, ecological transitions, longitudinal gradients, and patterns of biodiversity in stream networks.