Southern Guam geospatial information server
The land surface in southern Guam is very diverse. There are hills and lowlands, peaks and ridges, river valleys, coastal areas, etc. Those natural surface features are called topography. Surface features very much influence the nature of a place. They affect the vegetation, drainage, coastline, and even the weather.
When it rains in southern Guam the water flows down mountainous slopes and forms small gullies. Gullies merge into streams, and streams become bigger and bigger because they receive water from other streams that join them. Streams that join other streams are called tributaries. Streams that become quite big and have a lot of water are called rivers. Streams and rivers flow downhill and eventually reach the ocean. The place where a river flows into the ocean is called a river mouth.
Illustrations above from "Student Atlas of Guam" by Danko Taborosi and David T. Vann, published by BessPress, 2007.
Water rains down, runs into rivers, and flows into the ocean year after year, yet the rain and rivers never run out. This is because of the water moves in an endless natural cycle. Driver by the heat of the sun, the water in oceans, lakes, rivers and other water bodies warms up and evaporates. It changes from liquid form to a gas called water vapor. As the vapor cools down in the air it condenses to form clouds. When the clouds contain too much condensed water, droplets fall from the sky and create rain. This process is called precipitation. Rain falls back into the ocean and lakes (or onto the land surface and flows into the ocean and lakes). Some of it seeps into the ground and becomes groundwater, which also eventually makes its way into the ocean. Water from the ocean, of course, continuously evaporates and follows the never-ending water cycle.
Diagram above modified from USGS.
A watershed is a land area from which all the water drains toward a common destination (such as a stream, lake, or ocean). It includes both the streams that convey the water as wall as the land surface from which water drains into those streams. Watersheds are separated from each other by natural barriers such as ridges and mountains, which are known as drainage divides. Within each major watershed lie sub-watersheds or tributaries that drain into these larger watersheds.