It is forced to flow downhill by the Earth's gravity.
Downhill.
Water flows downhill, and the topography of that region just happens to be downhill to the east.
Rivers, lakes, canals, even sewers are bodies of water that flow downhill in a channel.
Gravity, rivers flow downhill
Streams form when downhill running water creates a channel capable of carrying water. Most streams begin as temporary rivulets that only have water after a rainfall.
river
river
Surface water flows downhill or seeps into the ground becoming groundwater. The natural flow of surface water is downhill or seaward. See the related link for more information.
Down hill, Think of it, Niagara Falls, the water is falling downhill, Niagara's water comes from St. Lawrence Seaway.
water can only flow oneway - downhill
A steep gradient will force the water to flow downhill faster.