In this paper a cross-layer design of joint congestion control and power control is proposed for wireless sensor networks (WSNs). The multiple access communication for the WSNs is assumed to be ultra-wide bandwidth (UWB) time-hopping spread-spectrum (TH-SS) impulse radio. The joint congestion and power control is formulated as an optimization problem which is then solved by distributed iterative algorithms for flow rate adaptation and power adaptation on every link. The transport layer controls the congestion using a hop-by-hop flow rate assignment. Power control algorithm allocates the right amount of power at the right nodes to alleviate the bandwidth bottlenecks by increasing capacity, while maintaining the signal to interference-and-noise ratio above a given threshold for every link. Simulations show that considerable improvement in throughput can be obtained while also transmitting at lower power levels.