Which human activity is known to significantly impact river systems?

Deforestation is known to significantly impact river systems as it alters the natural water cycle and increases the risk of flooding and erosion. When trees and vegetation are removed, there is less interception of rainfall, leading to increased surface runoff. This can result in a higher volume of water flowing into rivers after a rainstorm, which may exceed the river's capacity and cause flooding. Additionally, the removal of plant roots, which normally stabilize the soil, can lead to increased soil erosion. This erosion contributes to sedimentation in rivers, which can degrade water quality, disrupt aquatic habitats, and alter river morphology.

In contrast, urban landscaping, reforestation, and agricultural development each have impacts on river systems, but they do not have the same immediate and pronounced effects as deforestation. Urban landscaping can affect drainage patterns, but its effects are typically localized. Reforestation usually has a positive impact by restoring natural vegetation, and while agricultural development can lead to issues such as pesticide runoff or nutrient loading, the systematic removal of trees and associated soil impacts seen with deforestation presents a more critical threat to river systems.