Sediment Source Assessment

Background

Stream sediment results from erosion that may be part of a natural process or one influenced by cultural activities. It can be deceptively difficult to identify sediment sources and to distinguish natural from human-caused sources. Potential sources of erosion and sedimentation can be visually identified as gullying, landscape scars, road and bridge washouts, and risky cultural practices such as till agriculture on steep slopes. The relative risk of erosion can also be predicted as a function of soil type, slope and other factors.

Findings

No comprehensive data on sediment sources in Rock or Richardson Creeks were available for this assessment. Moreover, the ongoing stormwater study commissioned by Clackamas County Water Environment Services is not intended to comprehensively address erosion or stream sedimentation.

Stream sedimentation is a significant issue in parts of these watersheds, and high levels of silt, sand and fine organic matter may adversely impact successful spawning of both salmonids and lamprey in some areas (ODFW, 1999). Excess sedimentation was observed in lower Rock Creek, and is considered to be one of several factors creating sub-optimal salmonid habitat below the falls in Reach 1 (Fowler and Davis, 2000). Limited field observation indicates gully and surface erosion occurring in the steeper slopes of the Prairie Terrace zone west of Damascus currently under till agriculture, and in the steeper road-side ditches in the upland areas of Rock creek watershed as seen along Foster Road. Bank erosion and silt deposition is particularly significant in Reach 5 of Rock Creek (Wolfe, K. et al, 1999). Bank erosion in this particular portion of Rock Creek is degrading potential spawning and rearing habitat for cutthroat trout. Bank instability has been aggravated by the removal of riparian cover in this reach. Some bank erosion is also apparent in Upper Richardson Creek. The lack of adequate riparian buffers and vegetation in the upper reaches of both watersheds poses a risk of future sedimentation.

Riparian Zone Vegetation Characteristics by Watershed *
Land cover	Rock Creek		Richardson Creek
	acres	percent total	acres	percent total
Urban / built up	121	9.10	58	9.30
Agriculture
Low structure agricultural	316	23.88	100	16.10
High structure agricultural	14	1.03	27	4.40
Forest
Closed canopy deciduous	239	18.02	38	6.11
Closed canopy mixed	261	19.70	215	34.41
Closed canopy coniferous	28	2.11	20	3.18
Open canopy deciduous	56	4.20	39	6.33
Open canopy mixed	74	5.58	18	2.83
Open canopy coniferous	3	0.22	1	0.17
Deciduous scattered	44	3.29	20	3.15
Mixed scattered	20	1.49	8	1.24
Coniferous scattered	3	0.22	0	0.06
Shrub
Closed canopy shrub	40	2.99	16	2.51
Open canopy shrub	18	1.36	8	1.26
Scattered shrub	23	1.71	9	1.51
Grassland, golf courses, parks, meadows	67	5.06	46	7.41
*Total (riparian zone)*	1,324		623
*riparian zone based on 200 foot buffer on all major streams. Source: Metro (1998).

Soils and elevation were used to develop the map of Steep Slopes and Erosion Hazards, showing slopes greater than 25 percent and soils rated with a high erosion potential by the Natural Resource Conservation Service. The lower canyon areas of both Rock and Richardson Creeks include considerable overlap between steep slopes and high erosion potential soils. The mid-slope of the Boring lava domes in northeast Rock Creek watershed and northern Richardson watershed are also characterized by steep slopes.

The potential for stream sedimentation in both watersheds to increase in the future is quite high. A model developed by URS Corporation to estimate pollutant and sediment loads under future urbanization indicated a six-fold increase in suspended sediment loads (Wu and Fowler, 2000).

The channel condition assessment in the URS Corporation natural resources technical memorandum provides some specific information on individual reaches and tributaries in both Rock and Richardson Creek relative to potential sediment sources (Wolfe, K. et al, 1999, pgs. 5–9).

Information gaps

Empirical data on erosion and stream sedimentation is lacking.
The relationship between observed erosion and stream sedimentation is unknown.
Ancillary data necessary to assess sedimentation, such as peak stream flows, debris flow potential and landslide inventories, are lacking.
The impact of new impervious surfaces and urbanization more generally on sedimentation has not been analyzed. Recommendations
Work with interested residents and landowners to identify visible erosion sites and cultural practices that may lead to sedimentation.
Work to educate residents, landowners, and developers on best management practices to limit erosion and sedimentation and develop local groups to monitor activities.
Work with appropriate planning agencies and landowners to protect areas with steep slopes and high erosion potential soils through maintenance and enhancement of stabilizing forest cover.
Encourage tree planting in sensitive riparian zones.

Riparian and wetland assessment

Background

Riparian and wetland areas are two of the most critical elements within watersheds that influence the health of the aquatic ecosystem. As mentioned in the History section, Douglas-fir forests characterized much of Rock and Richardson Creek watersheds prior to Euro-American settlement. Riparian areas, or the vegetation zone that influences stream conditions, varied in width depending on the stream channel type and surrounding topography. For the purposes of this analysis, we have assumed a uniform width of 200 feet on either side of streams, regardless of channel type.

Findings

Rock and Richardson Creek land use The characteristics of riparian vegetation within Rock and Richardson watersheds can be described in terms of three general geomorphic zones: (1) the upland lava dome and first order streams, (2) the valley floor second and third order streams, and (3) the lower creek canyon mainstems and floodplain.

The lava dome upland riparian zones are mostly forested, with second or third growth deciduous and coniferous trees in the overstory, mostly alder, maple, and Douglas-fir. Tree canopy ranges from 30 to 90 feet in height. Understory plants include hazel, dogwood, and blackberry brambles. In some areas, roads wind along these streams (Borges, Tillstrom) and thus disrupt the riparian cover. A utility corridor on Scouter Mountain cuts across the riparian zones of two upland creek sections. Culverted driveway crossings are numerous, particularly in the hills east of Pleasant Valley. In other areas, particularly in upper Richardson Creek tributaries, pastures and fields come right to the edge of streams.

The valley floor riparian areas are only moderately forested. Agriculture, subdivisions, golf course construction, and roads have heavily altered the valley floor riparian zones. Generally, these areas are characterized by a narrow (10 to 30 feet) band of shrubs and small trees (willow, dogwood, ash, and blackberry). Sections of valley streams have been channelized, further impacting the riparian zone. Some areas have no riparian cover at all, including parts of the golf course, the stream that flows through Damascus, and several stream sections in Pleasant valley. Other areas have orchards, nurseries, and berry fields crossing right over streams.

The lower creek canyon mainstems of both Rock and Richardson Creeks are mostly forested. This forest is fairly mature, and even includes individual old growth conifers, as well as cottonwood, alder, maple, and oak. The riparian zone of lower Richardson seems to be of higher quality than that of Rock, but both are in fairly decent condition. The floodplains along the Clackamas, at the mouth of both creeks, also have mostly forested riparian zones.

The table below showing riparian zone vegetation is a summary of vegetation types within 200 feet of all streams in the two watersheds. The 200-foot zone is an arbitrary estimation of the true riparian area. The land cover map data used for this analysis is designed for regional analyses and is not appropriate for assessments of stream shading or in-depth local riparian characteristics. Nonetheless, this broad analysis provides a rough picture of riparian conditions. The map of Land Use and Land Cover shows the distribution of broad vegetation and cover types across the entire landscape.

The URS Corporation natural resources technical memorandum provides some brief descriptions of riparian conditions by reach based on limited visual observations in both Rock and Richardson Creek (Wolfe, K. et al, 1999, pgs. 2–5).

More detailed riparian condition data will soon be available pending completion of Metro's regional riparian mapping project, which will assess vegetative and land use conditions along streams and rivers in analysis units directly tied to stream channel types. The resulting inventory and database is intended to support Metro's Title 3 fish and wildlife programs and meet Statewide Planning Goal 5 inventory standards. Rock Creek watershed is the pilot area for this project.

Wetlands

Rock and Richardson Creek hydric soils Historically, wetlands of Rock and Richardson Creeks were also likely dominated by forests, primarily ash, alder, cottonwood, and red cedar. The historic distribution of wetlands in this area is unknown. Maps showing the present distribution of wetlands come from the National Wetlands Inventory (USFW) and from Clackamas County Water Environment Services. Some wetland delineation was done as part of the Sunrise Corridor environmental impact statement.

Wetlands occur primarily as small patches located on the valley floors and along the floodplains of both creeks. Most are adjacent or near streams. They range in size from little more than 1,000 square feet to 12 acres, the largest being located on the floodplain of lower Richardson Creek. This shows on a 1961 USGS quad map as Lake Pigeon. It appears to be a grassy swamp, or bog surrounded by shrubs and forest. It may be intermittently flooded, and provide refuge habitat for salmonids and other aquatic wildlife.

Several wetlands appear to have been diked, drained, or otherwise altered as a consequence of housing development. Clackamas County has identified a cluster of four wetlands just east of the Highway 212–224 junction.

A number of artificial ponds have been created. There are several in the middle Richardson Creek area, south of Damascus. The golf course in northern Rock Creek watershed also has created at least two ponds. Another can be found along upper Rock Creek.

The Pleasant Valley floor has a large area of hydric soils that may indicate former wetlands. These are presently pastureland, and show sedges and other signs of surface water in winter. Interviews with long time residents in upper Pleasant Valley (Kelly Creek Watershed) indicate that the water table may have risen in this area as a consequence of removing the forest cover (Olson, et al, 2000).

The location and distribution of known wetlands and hydric soils is shown on the accompanying map.

Information gaps

Riparian vegetation has not been classified or related to Channel Habitat Types.
Riparian vegetation has not been broken into condition units.
Stream shading has not been measured or analyzed.
Field verification of riparian conditions, particularly on valley floors and in the lava domes, is lacking.
Reference ecosystems to guide restorative work have not been developed.
The link between hydric soils and former wetlands has not been confirmed.
The condition of existing wetlands has not been analyzed.
The ecological role of wetlands in these watersheds is not fully understood. Recommendations
Coordinate with Metro's ongoing regional riparian mapping project designed to assess vegetative conditions and land uses along streams and rivers.
Develop further analysis to establish desired widths for restored functional riparian areas throughout each watershed.
Research the historic presence and condition of wetlands in these watersheds.
Research the role of hydric soils in storing water for summer base flows.
Work with private landowners and community to protect and enhance wetland areas and seek to expand functional wetlands in areas with hydric soils.
Consider wetland enhancement and stream protection below Foster road to include critical cutthroat habitat.