The Five Elements of Urban Stream Functions

Defining The Five Elements

• Community Values

  Communities cherish streams for their strong sense of place in the urban environment, granting waterway managers broad discretion to restore their function. Recognizing community values, including social benefits and concerns, guides the selection of assessment metrics for urban streams. Neglecting these connections may lead to outcomes with little value to people or even to conflicts between community values.

  In urban waterway design, MHFD works with communities to prioritize ecological health alongside access and amenities for the community. Identifying community-valued attributes requires an inclusive understanding of diverse community values and in some cases, social science expertise to know how to find those values. In emerging areas without established communities, stakeholders must advocate for future community rights to form meaningful relationships with streams, ensuring compatibility with functioning systems.

• Hydrology

  Runoff, a key factor influenced by urbanization, significantly impacts hydraulic, geomorphic, and ecological conditions. The Urban Stream Assessment Procedure (USAP) evaluates hydrologic processes crucial for stream resilience, animal and plant life, and human benefits. For example, overbank flows nourish floodplain vegetation, while moderate peak flows aid sediment flushing and channel maintenance, and baseflows provide persistent habitat and recreational opportunities.

  Urbanization impacts hydrology differently based on design and pre-development conditions. Peak flows and runoff frequency typically increase due to heightened runoff from impervious surfaces and sewer overflows. Effects on low flows vary; they may decrease from reduced groundwater recharge or increase from leaks and increased water use. Urban streams exhibit variable flow volumes often becoming flashier, though not universally in naturally flashy systems like arid lands.

• Hydraulics

  “Hydraulics” encompasses water movement in channels and floodplains, expressed in depths, velocities, and forces, as well as interactions between sediment, water, and wood. Alterations to flows, a result of hydrological processes, affect hydraulic dynamics. Water movement shapes stream geomorphology and vegetation across various spatial and temporal scales. Stream channel size, vegetation distribution, bed and bank stability, and aquatic habitat depend on flow regimes, local geology, and physical features. USAP aids practitioners in understanding these relationships to define flow requirements for urban streams, enabling the definition, assessment, and policy guidance for more functional flow regimes.

• Geomorphology

  Urban streams’ characteristics, including location, shape, and form, are shaped by geomorphic processes like erosion, sediment transport, and large wood dynamics. However, these elements can be altered directly through interventions like constructed channels or rock protection, influencing ecosystem functioning and geomorphic hazards. Human development has led to alterations in physical habitat and sediment transport, contributing to the degradation of urban stream systems.

  The connection between a stream and its floodplain is vital for riparian vegetation, habitat, and resilience, and this connection is often severed in urban stream systems by impacts related to urban development. USAP addresses this by focusing on key geomorphic processes such as erosion and sedimentation, recognizing their importance in urban stream management. Assessment of physical attributes at various scales helps us understand geomorphologic characteristics, their role in supporting human values, and their sensitivity to degradation or management interventions.

• Vegetation

  In urban streams with altered hydrology, improving riparian or wetland vegetation may face challenges due to the persistent effects of altered streamflow. Plant communities that originally established under one set of flow conditions may have trouble adapting to the changes in flow regimes resulting from urban development, and predicting the right vegetation type and location for a future development condition can be very challenging. Thus, rethinking our approach to assessing urban streams is crucial for ensuring the long-term health of vegetation in urban stream systems.

  Understanding the interactions between vegetation structure, hydrogeomorphic processes, and stream dynamics is essential. Riparian vegetation supports stability, flow resistance, sediment retention, and provides large wood, enhancing structural complexity. Urban streams not only deliver ecological functions but also support social values vital to communities. Therefore, assessing these functions is critical for identifying and implementing management actions to address stressors degrading them.