Middle States Geographer, 1998,31: 1-14 SPATIAL VARIABILITY IN STABILITY THRESHOLDS,

ABSTRACT: This study documents the pattern of spatial variability at ' channel stahilir\', as represented by stability thresholds, along a mid-sized dryland stream, the Verde River in central Arizona. Examining variations in stream competence (as represented by shear stress) and resistance (as represented hy the size o { sediments comprising various geomorphic surfaces) within the stream channel and along the entire river shed light on the nature o { variations in sediment transport. Under this shear stress approach to stability, a threshold exists, dividing conditions of stability and little sediment motion, from conditions of instahility and sediment mohility. Channel morpholoR) ' and sediment data, combined with calculations of discharge and shear stress, facilitate the creation o{ stahility thresholds for 14 cross-sections representinR reaches of the Verde Ril'er. Different Reomorphic surfaces exhihit different stability thresholds: most low-flow channels and flood plains are unstable at the discharge with a two vear recurrence interval, while only afew side bars are unstable. At moderate flows (discharges with a ten year recurrence interval), most of the surfaces within the study cross-sections are unstable, implying that within-channel shifiing and channel change may occur on average el'ery ten years. These results imply that during more frequellt, lower flow CI'ents, the sediment transport system of the Verde River is highly fraRmented, with localized areas o{ sediment entrainment and areas of sediment storage. During moderate 10 high flow events, however. the river is more inteRrated, variations in stability largely disappear, and sediment moves per/zaps more predictah/r throug