Many channelized streams have straight trapezoidal channels that lack geomorphological structure and physical habitat, such as pools and riffles. This chapter focuses on design and implementation of pool-riffle structures in straight channels through examination of three case studies: one in which pool-riffles have been implemented successfully, one in which implementation failed, and one in which implementation has yet to occur. Basic geomorphological principles indicate that a key element in the design of pool-riffle units is the need for strong flow convergence from riffles into the pools to maintain transport of sediment through the pools as discharge increases, thereby promoting removal of accumulated sediment. This idea is evaluated in the first case study through one-dimensional (1-D) and 3-D numerical modeling of flow through a basic pool-riffle design, along with field and laboratory measurements of flow through the designed pool-riffle structures to confirm the validity of the numerical modeling. The design was implemented successfully in a straight urban channel with limited sediment load. The second case study, where implementation failed, involved crude construction techniques that did not conform to design criteria and thus did not provide appropriate hydraulic conditions for pool scour under conditions of high sediment load, resulting in pool infilling. Lessons learned are being applied in a third case study, where the basic design has been modified to accommodate straight channels with high sediment loads. Modification involves enhancement of forced constriction of flow into the pools to produce appropriate hydraulic conditions for removal of accumulated sediment from pools at high flows.