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WENATCHEE RIVER CHANNEL MIGRATION ZONE STUDY

Summary
February 13, 2003 - Leavenworth Public Presentation

Study Objectives

The overall objectives of the Wenatchee River Channel Migration Zone (CMZ) study are to:

• Evaluate historic changes in channel behavior and vegetation for the Wenatchee River and some of its tributaries.
• Project areas where these rivers and streams may migrate or erode their banks in the future.
• Identify potential restoration sites to improve salmon habitat.

Study Area

The study area includes the Wenatchee River, from Leavenworth to the confluence with the Columbia River; the mouths of Icicle, Peshastin and Mission Creeks; and the lower four miles of Nason Creek.

Methods for Assessing Changes in Rivers and Streams

Aerial photograph interpretation, geographic information system (GIS) mapping, and review of previous reports have been used to evaluate changes in river and stream channels for the CMZ study. Key steps in the study have included the following.

1. Dividing the study area into reaches (segments) and developing an overall understanding of the streams by reviewing reports, maps, and aerial photos. The aerial photos used were from 1956, 1979, 1992 and 1998, covering 42 years of channel changes.

2. Preparing detailed maps of each reach using GIS. Key features from the aerial photos were digitized (converted into digital information for use in GIS mapping). These features included elements such as:

• Outer channel banks, bars, islands and water surface areas, logjams, side and back channels, sloughs, and channel thalweg (path of the deepest flow).
• Boundaries of the valley flat area, including the floodplain and lowest level terrace adjacent to the channel and geologically resistant features including terraces (raised or relict floodplain), hillslopes (high relief or mountainous areas), and fans (primarily colluvial fans).
• Evidence of human interference with the river and creeks including the railway, major roads, bridge crossings, the weir near Peshastin Creek, and levees. (It is assumed these structures are protected by loose rock or riprap where river erosion could threaten them.)

3. Calculating channel characteristics such as average channel width for each reach. Bank erosion, channel deposition, and loss of valley flat to development were calculated by overlaying maps from one year to the next.

4. Developing draft CMZ maps by projecting historic erosion rates into the future, using rates calculated at individual sites along the river. The overall process involved preparing maps showing changes in bank line, calculating the prevailing direction of erosion, calculating average erosion rates and extrapolating these rates to 50 years, then extending the predicted erosion rate from the 1998 bank line. The channel migration zone (CMZ) was then digitized.

5. Performing a draft hazard zone rating. A draft map was prepared by superimposing the island and valley flat information for each date of mapping using GIS and then classifying the severity of hazard based on observed or historic erosion. Areas that have no historic erosion were assigned a low hazard, while areas that were projected to erode over the next 50 years or that lie within the channel were assigned higher hazards.

Methods for Assessing Changes in Vegetation

The CMZ study also included an analysis of the changes to vegetation along the Wenatchee River and adjacent hillsides. Aerial photographs from 1979, 1992, and 1998 were used to analyze changes over time and identify potential causes for the change. Vegetation types were mapped in the areas immediately adjacent to the river, on valley flats next to the river, and 200 feet beyond the valley flat.

Study Results

Stream and River Channels

The Wenatchee River is partly incised or entrenched, has a narrow floodplain, and has maintained the same general alignment for at least 100 years. Now its banks are often stable, both because of bank protection and the presence of geologic formations that resist erosion. Part of the river's floodplain is no longer accessible because of development. Still, the Wenatchee and its tributaries erode their banks and their channels do shift, although generally at low rates.

Reach-averaged erosion rates were greatest from 1979 to 1992. The greatest deposition occurred from 1956 to 1979 as part of channel narrowing resulting from development and natural processes. Long-term average erosion rates ranged between 0.5 and 3 feet per year. Erosion was greatest in Reaches 1, 2, 4, 6, and 9 of the Wenatchee River and in the tributaries, particularly Nason Creek. The channel migration zone includes the high hazard erosion zones (those areas where projected erosion would occur over the next 50 years) and the very high hazard zones (areas such as the active channel and where erosion has occurred over the 42 years of this study).

Vegetation

As expected, human influence dominates the landscape for all three years studied, with orchards, fields, urban areas, and other human features.

Vegetation communities that are commonly linked to riparian (streamside) vegetation were found to be relatively constant throughout the study period. These include Vegetation Units E-I, and K as shown in Table S-1 below. These communities totaled 508 acres, 493 acres, and 500 acres for 1979, 1992, and 1998, respectively.

Table S-1. Area of Vegetation for 1979, 1992, and 1998 on the Wenatchee River

The amount each vegetation type changed over the study period and the likely causes for the changes are shown in Table S-2. The two most common factors resulting in a change in vegetation types were aging of trees and human activities. "Human landscape" features increased by 53 acres between 1979 and 1998.

Restoration Opportunities

After the river features and vegetation units were mapped, a fisheries biologist reviewed the maps and conducted a one-day reconnaissance site visit to identify potential restoration opportunities. Restoration opportunities included sites that could be selected for preservation, enhancement, or restoration.

• Enhancement means to increase one or more functions that an existing riparian or stream habitat possesses.
• Restoration refers to actions that can be performed to reestablish functions or characteristics of the habitat which have been lost.
• Preservation is recognized as a site where protecting the area can preserve the existing habitat value and allow ongoing natural processes to continue.

A total of 24 restoration sites were selected. The sites included areas that could be preserved because of their existing high-quality habitat adjacent to the Wenatchee River. Restoration and enhancement can take many forms, but the most common types identified in this study include:

• Creating off-channel habitat
• Providing peak flow refuge for salmonids
• Developing riparian vegetation

Reconnecting floodplains, back channels, and sloughs are techniques that can be used to improve habitat conditions.

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