Usaer Mission Creek Terrain Stabutv Study

Activity 704791,

March 2000,

Westrek Gestee hnical Services

V a a i n e Geola~ieal Enerineerine Limited

This study was initiated by Riverside as part of the total resource-planning project for Upper Mission Creek area and to address concerns relating to the high level of suspended sediment and bedload in Mission Creek following the 1997 freshet and the extent of channel disturbance through private land downstream.

Mission Creek is located in the western portion of the Okanagan Highland and is the largest tributary to Okanagan Lake. The main-stem of Mission Creek, from Okanagan Lake to the confluence with Fish Hawk Creek, is approximately 60 km in length. Fish Hawk Creek, from the confluence of Mission Creek to Fish Hawk Lake is approximately 12 km in length. The estimated size of the Mission Creek Watershed is 816 krn2. Mission Creek enters Okanagan Lake near the southeast end of Kelowna.

A total of 95 active and inactive landslides and erosional features were identified within the lettered zones in the study area. The landslides and erosional features are categorized as debris slides,debris slumps, bedrock slumps, erosional undercutting and a combination of debris slides and erosional features. Anecdotal evidence and visual observations confirmed the actual occurrence of landslide events in the springs of 1974 and 1997. The majority of these events are in varying stages of revegetation. Several of these features displayed episodic behavior with almost 100% revegetation observed in the upper segments and 0 to 20% revegetation in the lower segments.

Results from the historical air photo analyses from 1 939 to 1 997 conclude that the landslide and erosion events observed within the detailed study area during this study, are part of the natural processes involved in the formation of this valley since the last period of glaciation. 97% of the events identified in Zones A to L are naturally occurring. The majority of the disturbance (approximately 75%) noted during this study was in un- logged portions of this watershed.

A review of the hydrologic and climatic data for this study and surrounding areas, during these periods, confirmed the following: -

9 The groundwater level in 1973 was the highest on record, which suggests that antecedent moisture conditions in 1974 were likely to be higher than average. The snow pack in 1974 was the 2nd highest snow pack on record, while the spring snowmelt was the 1 5b highest on record. The annual accumulated precipitation was 7" highest on record. The maximum instantaneous peak discharge was 51h

highest on record.

P 1997 had the 4a highest snow pack on record and the 7h highest spring snow melt rate. No groundwater data for this year was available to complete the project. The 2"d highest maximum instantaneous peak discharge was recorded on May 3 1'' 1997. The annual accumulated precipitation ranked 8Ih on the record for this year.