Why does the Brahmaputra in its Tibetan part have less silt, despite a longer course?

The Brahmaputra River, in its Tibetan part, has less silt despite its longer course due to various factors. It flows through rocky terrain with limited vegetation, resulting in less soil erosion and silt. Moreover, its origin from glaciers produces cleaner, less sediment-laden water. Additionally, minimal human interference in the Tibetan region reduces human-induced soil erosion. However, as it travels through lower-altitude plains downstream, it gathers more silt due to softer soil, agricultural activities, and increased human influence.