QUANTIFIED CHRONIC SEDIMENT LOAD FROM ROAD CONSTRUCTION IN GARRAPATA WATERSHED, A STEEP COASTAL TROUT STREAM ON THE BIG SUR COAST, CA

Garrapata watershed lies immediately south of the Carmel watershed in the Santa Lucia Mountains of central CA. It is one of the last remaining steelhead bearing streams in the south/central evolutionary significant unit. This study analyzes a three-year record (2001-2004) of water and sediment discharge from Garrapata Watershed (27.5 km²) and one of its tributaries, Joshua Creek (5.3 km²).

Garrapata is a perennial, steep (12%), high-relief (915 m) watershed underlain by highly erodible, fractured granodiorite. Average rainfall is 74 cm/a of rain, chiefly during intense winter frontal systems with a strong orographic effect. The land use is sparse homes and dirt roads within a natural mosaic of chaparral, grassland, and woodland. The watershed was logged a century ago, but no significant sediment sources remain from that era.

A poorly placed dirt road in Joshua Creek has become a chronic sediment source in the watershed. Gullies, landslides, and talus aprons from the road cut and side-cast prism are liberating sediment and delivering sand and fine-grained material along a 2 km length of Joshua Creek. A comparison of the sediment loads in Joshua Creek and Garrapata Creek allows the impact of road construction to be quantified.

We sampled bedload 9 times over three years with flows ranging from 0.001 m³/s to 0.25 m³/s. The sediment rating was analyzed through time using a continuous water discharge record. The average annual flow of sediment (mostly sand and small gravel) from the Garrapata watershed was 190 m³/a (507 tonnes/a), while Joshua Creek accounted for 150 m³/a (400 tonnes/a). Therefore, Joshua Creek produced 78% of the total annual sediment, yet accounts for only 19% of the Garrapata Watershed area. On average, Garrapata Watershed (excluding Joshua Creek) carries 56 g of bedload per m³ of water, whereas Joshua Creek transports 1040 g/m³.

Joshua Creek has a marked loss of pool habitat. Based upon extrapolated field reconnaissance, we estimate that there were approximately 584 tonnes of mobile excess sand residing in the pools of Joshua Creek in early November 2004. Using our sediment transport data, we calculate that Joshua Creek channel would be clear of the pool-filling sand and silt within approximately 1.6 years if all sources of excess sediment in Joshua Creek were suddenly stopped.