Climate change is a major cause of the increase of natural disasters like floods, droughts, and storms. Although several countries have been affected by such natural disasters, the East African region is one of the most affected. This work focuses on floods as the most frequent disaster in the East African region where 280 died and about 2.8 million people were affected by the floods that occurred in 2019 alone. During the floods, people not only lost their lives and got displaced, but property and infrastructure were also damaged. Different techniques have since been developed to mitigate the effects of floods. However, the methodologies used have not responded to the identified problems that include lack of community awareness, information inadequacy, and low-cost systems. To solve these problems, the present study aims at developing a low-cost system that detects and alerts the community on upcoming flood incidents. The proposed floodwater detection and early warning system comprise of three units. The sensing unit continuously monitors environmental parameters using ultrasonic, temperature and humidity sensors. The processing unit processes and analyses the collected data from the sensors. Then, the alerting unit alerts the community and local authorities using a buzzer and a Short Message Service (SMS) notification. The system uses the global system for mobile communication to provide internet connectivity which enables data to be collected, stored, and monitored in the cloud. The system was implemented at Themi river as one of the case study areas. The results showed that floods can be detected and the community near the flood-prone area alerted early. Therefore, the use of the developed system in flood-prone areas can contribute to environmental disaster risk mitigation, and subsequently, the death rates due to environmental natural disasters.