The root of branching river networks.

Branching river networks are one of the most widespread and recognizable features of Earth's landscapes and have also been discovered elsewhere in the Solar System. But the mechanisms that create these patterns and control their spatial scales are poorly understood. Theories based on probability or optimality have proven useful, but do not explain how river networks develop over time through erosion and sediment transport. Here we show that branching at the uppermost reaches of river networks is rooted in two coupled instabilities: first, valleys widen at the expense of their smaller neighbours, and second, side slopes of the widening valleys become susceptible to channel incision. Each instability occurs at a critical ratio of the characteristic timescales for soil transport and channel incision. Measurements from two field sites demonstrate that our theory correctly predicts the size of the smallest valleys with tributaries. We also show that the dominant control on the scale of landscape dissection in these sites is the strength of channel incision, which correlates with aridity and rock weakness, rather than the strength of soil transport. These results imply that the fine-scale structure of branching river networks is an organized signature of erosional mechanics, not a consequence of random topology.

Rewards of reading: Toward the development of possible selves and identities.

Children's voluntary reading positively correlates with school grades, vocabulary growth, reading comprehension, verbal fluency, general information, and attitudes towards reading. Drawing on qualitative interviews collected alongside six waves of longitudinal survey data in an urban setting in eastern USA, We argue that voluntary reading by adolescents also provides learning opportunities that scaffold identity formation, afford 'spaces' where youth rehearse and relationally enact gender roles, ethnic/racial identification, and fashion educational aspirations. The interviews with African American and European American youth were conducted in five visits and spanned 3 years through senior high school and 1 year post-high school. Methods of inductive and narrative analysis identified patterns of benefits and potential drawbacks of voluntary reading. Amount of voluntary reading was affected by school, family, and social and work commitment pressures, and fulfilled a number of broad roles. What and how youth were reading was as important as the amount. Significantly, reading allowed adolescents to explore possible selves-an interest in historical figures helped one African American male to develop values resisting stereotypes of male or African American, just as an African American female came to resist conforming to gender and racial stereotypes in dress and occupational ambitions. Relationships between voluntary reading habits, family context and educational aspirations were identifiable for a number of the interviewees.