Assessing Clearcutting Activities: Measuring watershed effects

Introduction

How does clearcutting affect a region's watersheds?

Watersheds delineate the path of water runoff for an enclosed network of streams and rivers. The quantity and quality of water in these watershed networks is ecologically important because a host of animals, and in many cases people as well, depend on it.

Forestry activities, such as clearcut harvesting and road building, not unlike natural disturbances of wildfire, windstorms, and disease, can have significant impacts on a watershed. That's not to say the effects are always negative. That depends on the size, intensity, and spatial distribution of these events.

If tree-removing events occur widely across a watershed in a short span of time, you can expect an increase in stream flows, perhaps leading to flooding and increased sedimentation and general degradation of water quality. Trees, after all, are significant consumers of water in a watershed and have a large influence on the amount of runoff that reaches streams.

In municipalities or communities that rely on a watershed for water supply, the balance between nonforested and forested area is critical. For example, a large amount of clearcut area leads to rapid runoff and loss of water, while a lesser amount decreases water flow and its availability downstream. This balancing act becomes particularly critical in areas where spring snowmelt provides a large amount of water.

Road building is also an issue affecting watersheds. Careless building of roads across or near streams, or a high density of roads within a watershed, increase sediment loads and degrades water quality.

Location

A small 1400 ha woodlot in the Acadian-New England forest region of North America

Time to complete the lab

3 hours

Prerequisites

Students should have a basic working knowledge of GIS data, including use of a geodatabase. Some experience with rasters and the Spatial Analyst extension is also beneficial. Familiarity with basic forest analysis techniques is important, while familiarity with forest inventory data and terminology is also helpful, but not essential.

Data used in this lab

Feature classes and rasters for a forest woodlot (personal geodatabase)

Geographic coordinate system: NAD 1983 CSRS New Brunswick Stereographic

Datum: NAD83

Projection: New Brunswick Double Stereographic Grid (unless otherwise stated)