CONTRASTING STREAM STABILITY CHARACTERISTICS IN ADJACENT URBAN WATERSHEDS: SANTA CLARA VALLEY, CALIFORNIA B. A. JORDAN, a * W. K. ANNABLE, a,b C. C. WATSON a and D. SEN c a Department of Civil and Environmental Engineering; Colorado State University; 1372 campus Delivery; Fort Collins, Colorado 80523-1372, USA b Department of Civil and Environmental Engineering; University of Waterloo; Waterloo, Ontario, Canada N2L 3G1, Canada c Santa Clara Valley Water District; San Jose, California 95118, USA ABSTRACT A comparative study of two adjacent stream channels in the Santa Clara Valley region of California provided an opportunity to study the relative effects of multi-faceted watershed-urbanization impacts on channel evolution and stability. Berryessa Creek (15.5 km 2 ) and Upper Penitencia Creek (61.3 km 2 ) have similar intrinsic watershed characteristics; however, urbanization processes have imposed distinctly different evolutionary trends in each watershed. The influences of drainage network manipulation, hydrologic routing and engineering infrastructure has resulted in Upper Penitencia Creek remaining relatively stable throughout the course of urbanization, while Berryessa Creek has experienced system-wide channel instability problems. This study enumerates the many anthropogenic impacts and provides insight into basin alterations that can have either positive or negative feedbacks in maintaining or degrading channel stability throughout the course of urbanization. Results show that infrastructure that disrupts the bed material sediment continuity (such as large drop structures or sedimentation ponds) generate long-term downstream channel instabilities leading to channel degradation and continued maintenance. Off-line flow diversions (in this study percolation ponds) that do not disrupt bed material transport can emulate pre-urbanization conditions offsetting channel degradation resulting from changes in hydrology. This study also demonstrates the degradational responses of a stream due to losses in riparian vegetation from water table lowering transforming a perennial stream into an ephemeral stream resulting in increased bank instability. The importance of maintaining floodplains for flood access and channel stability has also been identified and compared to conditions of channel encroachment to facilitate maintenance, which have further exacerbated downstream channel degradation, long-term channel maintenance and dredging. Copyright # 2009 John Wiley & Sons, Ltd. key words: urbanization; fluvial geomorphology; channel stability; sediment transport; watershed analysis Received 23 March 2009; Revised 24 August 2009; Accepted 13 October 2009 INTRODUCTION The deleterious effects of watershed urbanization on stream-channel stability and riparian corridor health have been well documented (Wolman, 1967; Leopold, 1968; Hammer, 1972; Morisawa and Laflure, 1979; Booth, 1990; Booth and Jackson, 1997; Trimble, 1997; Chin, 2006). Hydrologic responses to urbanization typically involve the increased magnitude and frequency of peak flow rates (Leopold, 1973; Hollis, 1975) and decreased lag times (Leopold, 1968, 1991; MacRae and Rowney, 1992). Both washload and bed-material load correspondingly increase during the period of land-use change and the ensuing channel-adjustment period (Wolman, 1967; Leopold, 1973; Graf, 1975), which can often result in deposition in downstream reaches or overbank areas requiring long-term maintenance. After development maturity, washload supply is typically reduced due to paving and residential landscaping and bed-material supply is reduced due to channel bed and bank armouring. A common fluvial response to the hydrologic and sediment regimes altered by urbanization involves channel incision and accelerated bank erosion resulting in channel enlargement (Hammer, 1972; Neller, 1988; Gregory and Downs, 1992; Trimble, 1997; Pizzuto et al., 2000; Booth and Henshaw, 2001). Channel adjustments often lead to RIVER RESEARCH AND APPLICATIONS River Res. Applic. 26: 1281–1297 (2010) Published online 8 December 2009 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.1333 *Correspondence to: B. A. Jordan, Colorado State University; Department of Civil and Environmental Engineering; 1372 Campus Delivery; Fort Collins, Colorado 80523-1372, USA. E-mail: hydrogeodesigns@gmail.com Copyright # 2009 John Wiley & Sons, Ltd.