How Watersheds Work

What is a Watershed?

A watershed (or basin) is the area of land that catches snow and rain and drains it to a larger body of water, such as a marsh, lake, stream or river. Topography defines the entire watershed, as it shapes the course and speed of water moving through the area.

Watersheds can range in size from a few hectares to thousands of square kilometres (the Alberta part of the Battle watershed is 25,000 km2). All watersheds flow from headwaters (the headwaters of the Battle watershed come from Battle Lake) to outlets (the Battle River empties into the North Saskatchewan River), eventually ending in the ocean (Hudson’s Bay).

Watersheds fulfill three primary functions: to capture water, filter and store it in the soil then release it into a water body.

Watersheds sit on top of groundwater. Surface water (rivers, lakes, wetlands, etc.) and groundwater are connected in a watershed. Surface water seeps through the soil, moving downward to fill the cracks and spaces between rocks and soil particles, thereby becoming groundwater. Groundwater is stored in the soil and rock. Many people believe that groundwater comes from fast flowing underground rivers and lakes. This is not true.

Groundwater is always naturally in motion. Recharge areas are places where surface water soaks (infiltrates) into the soil to become groundwater. Discharge areas are places where groundwater seeps or flows into surface water (e.g. springs). The Battle River is fed by many springs along its length.

Run-off collects pollutants as it flows across land in a watershed. Polluted run-off flows into rivers, creeks, wetlands and lakes. It also seeps through the soil into groundwater.

Wetlands and riparian vegetation (plants growing alongside rivers, lakes, etc.) are like giant sponges. They store water, filter out pollutants and diseases, and slowly release the water into groundwater and/or rivers, creeks and lakes.

Watersheds are healthy when:

• land and therefore run-off is unpolluted
• there is plenty of vegetation to bind soils
• there are many wetlands and riparian areas to help clean and store water
• surface water can flow naturally over flood plains and from headwaters to outlets.

Humans affect the health of watersheds in their everyday lives, when they remove trees and plants, damage riparian vegetation, fill wetlands, pollute land or water, build homes or cultivate floodplains, create impervious surfaces (paved roads/parking lots), use large amounts of water, or change the flow of water with dams, weirs and culverts.

For more information

Getting to Know Your Local Watershed

Understanding Groundwater

Groundwater is the water that fills the cracks and spaces between soil particles, sand grains and rock. Each drop of rain that soaks into the soil moves downward to fill these spaces and gaps, thereby becoming groundwater stored in the soil and rock. Imagine a bucket filled with pebbles, then filled with water. The pebbles represent soil particles. The water between the pebbles is the groundwater.

Many people believe that groundwater comes from fast flowing underground rivers and lakes. This is not true.

When there are many interconnected cracks and spaces between soil particles and/or rock, we can easily pump out the groundwater. We call these zones aquifers. Sometimes aquifers can be easily mapped and predicted by geologists and drillers, but sometimes not. Imagine the bucket filled with water and pebbles. Push a straw deep into the pebbles and suck – the bigger the pebbles (and therefore the spaces between them) the easier it is to suck water out through the straw.

Groundwater is always naturally in motion. Recharge areas are places where surface water soaks (infiltrates) into the soil, thereby becoming groundwater. Discharge areas are places where groundwater seeps or flows into surface water (e.g. springs). Groundwater flow is part of the natural water cycle.

Pumping groundwater sucks groundwater out of the spaces between rocks and/or soil particles (i.e. out of storage). As water is pumped out of an area, ground and surface water from other areas (eg. neighbouring soils, wetlands, rivers) is slowly sucked in to fill the drained spaces in the rock and soil. Once again, imagine the bucket of pebbles filled with water. Put a bowl with a hole in the bottom on top of the pebbles. Fill it with water. This represents a lake or wetland. Then push a straw deep into the pebbles and suck. Watch the water in the bowl eventually drain away.

But because groundwater systems change very slowly, the effects of pumping can often take years to decades to observe. But in other cases it may only take days to months. It all depends on the geology.

Because there is so much water moving in the water cycle as precipitation, runoff, infiltration, evaporation, river flow, etc., it can be very difficult to measure the impact of pumping on water tables, surface-water levels, etc.

This page is based on information provided by Kevin Parks with Alberta Geological Survey (http://www.ags.gov.ab.ca/)