[MUSIC] Hydrology or the study of water is one of the most integrative topics we're going to look at in this course simply because all life depends on water. Hydrology is the science that looks at understanding, describing, and predicting how water moves through the landscape and through the atmosphere. In previous lessons we've considered hydrology in the context of climate. Later on, we'll explore glaciers, one of the most important features of the hydrologic water cycle in the mountains. And we'll also look at hazards where water often plays a critical role. Water's also essential for plants and animals that live on mountain slopes and is a key factor when considering the ecological adaptations of many species in the mountain environments. In this lesson we're going to focus on the basics of mountain hydrology. Why should people care about mountain hydrology? What are the different forms of water that you can encounter in the mountain landscape? How does water move through the mountain landscape and how can water change mountain landscapes? A quick glance at the globe indicates that much of the world's population is intimately connected to mountain hydrology, even people living in far-away lowlands. Let's look more closely at some of the major rivers in the world. The Nile in Africa, the Rhine in Europe, the Indus in India and Pakistan, the Yangtze in China, Mekong in South East Asia, the Murray in Australia, Columbia and Mississippi Rivers in North America and the Amazon in South America. All of these great waterways flow for hundreds or even thousands of kilometers and can have hundreds of millions of people living on or near their banks. People in both cities and rural areas rely on these rivers, either directly or indirectly for their livelihood. And all of these great rivers have their source or headwaters in mountains. Humankind's major civilizations were born along rivers. Mesopotamia between the Tigris and Euphrates rivers in the Middle East, ancient Egypt along the Nile and the great Chinese civilizations along the Yellow and Yangtze rivers. And for good reasons. Regular flooding along river channels created fertile plains for farming, and water flowed year round. And the animals and fish that lived in and near the rivers provided a ready source of food. This is still true today, and in modern societies, rivers are even more in demand. In the past, rivers were often the only ways to travel through the vast interiors of the continent. And many rivers are still fundamentally important navigation channels for transporting goods. As populations have grown and agriculture has become more intensive, reliance on rivers for irrigation is more pronounced than ever. Most major cities are on or near rivers, and their populations rely on those rivers for drinking water, regular household use, and industrial applications. These rivers' gradients or slopes are quite steep near their headwaters in the mountains, which makes them natural targets for building hydroelectric dams. For example, Canada's mountainous province of British Colombia meets nearly 90% of its energy demand through hydroelectricity. As civilizations have expanded, the demand on rivers has also grown. A good example of the incredible pressure on these rivers is the Colorado River. Which flows out of the central Rocky Mountains, through Colorado, Utah, Nevada, Arizona, California, and into Mexico. This river flows through an extremely arid region, with few other sources of water, but with intensive agriculture, and growing urban populations. The Colorado River is dammed for most of its length. And almost every drop of water is allocated to different states for different usages. To the point that 90% is diverted before it reaches the Mexican border. Today, this river rarely reaches its delta in the Gulf of California. While much of the world's economic well being depends on rivers that are born in the mountains, there are other aspects to mountain runoff that make it important to humanity. These are places of extraordinary beauty and many rivers hold a spiritual value for human societies. The Ganges River that flows through India and Bangladesh is the most scared of rivers to all Hindus. Mountain rivers are also centers for recreational activities such as canoeing, kayaking, whitewater rafting, and sport fishing. Rivers can also be a source of hazards. Mountain rivers, and streams, can experience flooding, and debris flows, which are fast moving masses of saturated rocks, and sediment, and organic material. Flooding can also impact the communities, well beyond the mountains. For example, in the spring of 2013, the city of Calgary in Alberta, experienced a devastating flood over the course of 24 hours with subsequent insurance damages exceeding $5 billion. Scientist often refer to mountains as the water towers of the world. It's a good metaphor. Water towers are essentially large, elevated tanks of fresh water. They store, and provide a usable source of fresh water for people that live near them. Mountains store water as ice and snow. And rivers that flow from the mountains provide fresh water downstream to populations and environments. Water is not only moved by the force of gravity, it also circulates through the atmosphere. In order to understand the importance of mountains as water towers, we need to first examine the hydrologic cycle. It may seem somewhat obvious why we should care about the hydrologic cycle. After all, water is critical to all life on Earth. But why specifically should we be interested in the hydrologic cycle in mountains? When we begin our journey through the hydrologic cycle, it's not uncommon to start with a water molecule evaporating from the ocean. However, you can just as easily start in the mountains where the water vapor in the atmosphere is forced to condense, falling to the ground as rain or snow. Water that starts at high elevations eventually flows to lower elevations. So, we'll begin by looking at mountains from this perspective, with mountains as a starting point for the hydrological cycle. Rain, snow, glacial melt water and ground water originating in mountains all end up in place, rivers. Rivers transport the water from mountain ranges to the lowlands and back to the seas and oceans. People are dependent on this water for their livelihood. One challenge for society will be to determine how these different sources of water might change patterns of water availability into the future. We often think of terrestrial and aquatic environments as independent systems. But they're not, in fact they're highly connected. The land that drains into a common water body is called a watershed. And healthy watersheds perform important regulating ecosystem services that influence water quality and quantity. The soils in a watershed act as a sponge soaking up precipitation in a process called infiltration. Vegetation in watersheds increases infiltration because it slows surface runoff, allowing more time for water to seep into the ground. By reducing the rate at which water washes down slope over the land, infiltration minimizes the erosion and prevents flash floods. Some of the water that is absorbed into the soil replenishes ground water stores. Which in turn, recharge lakes and streams. Therefore, watersheds not only slow runoff during precipitation events, but also regulate the release of water. This process also helps to maintain consistent stream flow and water supplies during dry periods. Watersheds also play a crucial role in cleaning and filtering our water supplies. As water moves through the soil, microbes remove harmful pollutants, toxins, pesticides and heavy metals, preventing them from degrading waterways. Likewise, plants purify water by taking up excess nutrients, including those released from fertilizers. Forests are also important for removing sediment from runoff. As water moves through watersheds, silt particles settle out. Because pollutants often stick to silt particles, this process of sediment deposition may further improve water quality. Society often neglects water resources. The natural systems at work in mountain watersheds are not easily replaced. For example, in the Catskill Mountains of the eastern United States, forests that provided purified water for the people in New York City were cut down through the 1800s to expand agricultural and urban areas. Combined with increased agricultural pollution and sewage, the removal of trees greatly deteriorated the quality of water available to New Yorkers, so the city was forced to investigate ways to fix the problem. They discovered that the estimated costs of installing a water filtration plant as a substitute for the natural ecosystem service was between six and eight billion dollars. Instead, they decided a much better long term solution was to purchase land and restore the free natural eco system service. This example provides an important reminder of our reliance on the critical regulating ecosystem services provided by mountains in our day to day lives.