Focusing on its characteristics:
* The river, which flows through the valley, is wide and deep. (Describes the river's location and size)
* The river, whose waters are crystal clear, is a popular spot for fishing. (Describes the water's quality and use)
* The river, that meanders through the countryside, is a source of life for many animals. (Describes the river's path and ecological role)
Focusing on its history or significance:
* The river, which was once a major trade route, is now a popular tourist destination. (Describes the river's past use and current popularity)
* The river, that inspired countless legends and stories, holds a special place in the hearts of locals. (Describes the river's cultural significance)
* The river, where ancient civilizations once thrived, is a testament to the power of nature. (Describes the river's historical importance)
Focusing on its impact:
* The river, whose floods can be devastating, is also a vital source of irrigation. (Describes the river's potential for both harm and benefit)
* The river, that has been polluted by industrial waste, is in desperate need of clean-up. (Describes the river's current state and need for action)
* The river, which sustains the local ecosystem, is a crucial part of the environment. (Describes the river's role in maintaining biodiversity)
Remember: Relative clauses add information about the noun they modify. You can choose different relative pronouns (who, whom, whose, which, that) depending on the type of information you want to add.