Direct measurements of secondary currents at a river inflexion point
SECONDARY currents are defined as currents which occur in a plane normal to the local axis of primary flow. They may develop in pipes or open channels as a result of non-equal distribution of boundary shear stress or by skewing of cross-stream vorticity into a long stream direction1–3. Secondary currents distort the distributions of primary isovels and boundary shear stress from those found in simple flows and have important effects on sedimentary processes of erosion and deposition4,5. Direct measurements of secondary currents in river bends were reported recently6. Further data have been obtained for the reach around the inflexion point between bends (Fig. 1) and are reported here.
Complex hydromorphology of meanders can support benthic invertebrate diversity in rivers
In freshwater environments, high biodiversity is commonly associated with habitat heterogeneity. River bends and meanders are particularly complex morphodynamic elements of watercourses. However, the specific spatio-temporal interactions between hydromorphology and the resident biota have scarcely been studied. This article reviews the relationships between hydraulic processes, and morphological units that are typical for meanders, and analyzes the concomitant spatial and temporal dynamics of habitats suitable for aquatic invertebrates. Flow in river bends is characterized by significant cross-stream velocities, which modify primary flow patterns, and create helical flow trajectories. Consequently, boundary shear stresses at the river-bed are altered, so that complex erosion, transport, and accumulation processes characteristically shape bed and bank morphology. The diversity of substrate types and complex bathymetry in meanders provide a large variety of habitat conditions for benthic invertebrates within a relatively small spatial domain, which are connected via hydraulic pathways. Periodic reversal of hydro-morphological processes between low and high flow, and seasonal growth of aquatic macrophytes creates spatio-temporal dynamics at the meso- and microhabitat scales. Such habitat dynamics increases benthic invertebrate diversity to the extent it is consistent with spatio-temporal scales of invertebrate mobility and life cycle. Furthermore, the presence of flow refugia, and hydraulic dead zones in meanders is essential to sustain species richness. This study concludes that meanders are highly complex morphodynamic elements that exhibit several self-regulating principles supporting invertebrate diversity and resilience in fluvial ecosystems.
Testosterone levels, handedness and sex ratio at birth.
Geschwind has hypothesized that levels of androgens in utero affect postnatal laterality preferences, high levels being associated with left-handedness of offspring. I have hypothesized that high parental androgen levels at the time of conception are associated with subsequent male births. However, left-handed parents seem to produce an excess of female, not male, offspring. This finding could be reconciled with the two hypotheses if individuals who were exposed to high levels of androgens in utero had low circulating levels in adulthood. Some evidence for this loophole is reviewed here. Lyster & Lloyd (1987, J. theor. Biol. 126, 125) have reported that (consistent with the hypotheses) index left-handed men have an excess of brothers; but, contrary to the hypotheses, index left-handed women have an excess of sisters. Accordingly these authors proposed that left-handedness in females is associated not with high levels of androgen in utero, but with a "sensitivity" to it. If such sensitivity were associated with low androgen levels, it might explain an excess of sisters among the sibs of left-handed women. It is shown here that evidence in agreement with this hypothesis (though not discriminating between it and Geschwind's) suggests that the sex ratios of offspring of LH male x RH female and of RH male x LH female matings, are both low.