Location Name: Mack Creek (Old-growth cross-section reach) - HJAMAC

Parent: Mack Creek - MACKCR

Landform:

The Mack Creek old-growth (MAC) and clearcut (MCC) sites are nearly adjacent, contrasting 320-m long reaches at the downstream end of the undisturbed portion of the Mack Creek watershed. The old-growth reach is fairly tightly constrained by adjacent hillslopes, particularly along the left bank, where there are several landslide scars in which large trees are absent and bedrock is exposed. The right bank is generally less steep and lacking in bedrock. Much of the right bank is bordered by a high, terrace-like landform, possibly residual glacial deposits. A 1st-order tributary informally named Devil's Club Creek enters from the right between XS 5 and 6. The tributary is incised into antecedent alluvial fan deposits. Bedrock is also exposed sporadically in the channel bed and banks within the old-growth reach, particularly where flow is directed against the bank as in the vicinity of XS 12 and XS 8 and 9 on the left bank (all of which are located at the foot of landslide scars and at the sharp left bend in the channel between XS 2 and 3. However, most of the bed within the old-growth reach is covered with alluvial sediment, even in areas with bedrock banks.

However, extensive areas of bedrock are exposed in the channel bed between the bridge and the upper end of the clearcut reach, especially within a narrow (7-m wide) boulder-armored section extending about 25 m downstream from the bridge. This boulder-armored channel was constructed in the summer of 1994, when the 10 ft (3 m) diameter, 75 ft (23 m) long culvert pipe previously at the road crossing was removed to improve fish passage. Approximately 1200-1500 yd3 of sediment were excavated at that time (Cissel, personal communication). Bedrock is also exposed in the channel bed within the two uppermost MCC cross sections (XS 101 and 102), but was not present at this location prior to the 1996 flood. No bedrock is exposed within the bed or banks of the clearcut reach downstream of XS 102, nor was any observed between the downstream end of the clearcut reach and the confluence of Mack Creek with Lookout Creek-a distance of approximately 700 m-during a reconnaissance trip in the summer of 1997.

Geology:

Woody debris is abundant within the channel, including many logs greater than 75 cm in diameter and a few as large as 1.5 m in diameter. Unlike Cold Creek, a majority of these logs rest on or partially on the channel bed rather than being suspended above the channel, due to the wider channel and valley floor. Many of the pieces less than 10 m in length have been fluvially transported at least some distance, and much of the LWD is concentrated into accumulations which span part or all of the channel. Within the MAC reach, there are two log steps and one large channel-spanning LWD accumulation (LWD jam); a second large LWD jam occurs just upstream of the uppermost MAC cross section (XS 12).

Vegetation:

The MAC reach flows through old-growth forest in which Douglas fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western redcedar (Thuja plicata) form a nearly closed canopy over the stream channel. Bigleaf maple (Acer macrophyllum) also grow within the riparian zone. Upstream of the road crossing, Mack Creek drains the largest uncut portion of the Andrews Forest. Its forest cover, due to the north-facing aspect, has experienced no fires in 450+ years and is unusually rich in old-growth relative other uncut areas of the Andrews (Weisberg, 1998). Hence, the MAC site is unique within the Andrews Forest and probably has few analogues in the Western Cascades.