Final answer:
The river that flows into Oxford, Reading, Windsor, and London, sourced from Gloucestershire, is the Thames River.
Step-by-step explanation:
The river that flows into Oxford, Reading, Windsor, and London, sourced from Gloucestershire, is the Thames River. The Thames River is the longest river entirely in England and passes through several major cities, including London. It has played a crucial role in the history and development of London as a major city.
London is located on the banks of the Thames River, and the river has been an important transportation route, source of water, and a feature of many cultural and recreational activities in the city. It is often referred to as the heart of London and has been immortalized in literature, poetry, and art.
In the poem mentioned, 'I Wandered Lonely as a Cloud' by William Wordsworth, he describes the river as the 'charter'd Thames', emphasizing its significance to the city.