Water around the world is separated into two categories: fresh water, and salinated (salt) water. The bulk of the Earth's water is salinated water, forming the oceans; only about 2.6% of all water on Earth is fresh, with 2% of that locked in the polar caps and glaciers, leaving about .6% in lakes, rivers, and groundwater. Because of the scarcity of fresh water, pollution is a serious concern.
Human pollution of water has always occurred, historically from cattle wastes and graveyards contaminating groundwater. After the Industrial Revolution, with steam-power and then petroleum power readily available, water pollution reached record highs; waste from factories were commonly dumped into rivers, and despite current regulations, rivers are still dumping grounds in many areas. China has one of the worst track records for river pollution, as they have few environmental regulations. A great deal of unconscious pollution comes from flush toilets, as people flush all sorts of things that clog pipes and do not degrade in the sewer systems.
Aside from chemical wastes, humans have also polluted waterways with trash and debris from various activities. Non-biodegradable products dumped into lakes and the ocean have clogged areas and killed off indigenous wildlife; the "Great Pacific Garbage Patch," while not consisting of a solid mass of trash one can walk across, is still a very destructive force in the ocean, as the currents collect floating trash which can and does kill fish and birds.
As the population of the Earth increases, we need to focus on renewable resources rather than single-use products, and we also need to improve our recycling and re-usage capabilities, which today are woefully inadequate.
Just to list a few reasons: fuel that is leaked from our boats etc. and trash that we leave behind. Below is a link to American Rivers that may give your more helpful information.