Restoration Program

Program Administration

Riparian Vegetation

Riparian vegetation downstream from Lewiston Dam has encroached significantly since completion of the TRD in 1964. With only 150 cfs to 250 cfs released year-round through the 1970s (except occasional, higher dam safety releases), seedlings and saplings escaped desiccation and (or) scour. These conditions significantly reduced scouring, and virtually constant instream flows impacted channel morphology and the river ecosystem by allowing woody riparian vegetation to rapidly encroach across the former active channel and down to the edge of the low-water channel.

	Typical fossilization of a point bar surface (circa 1995) near Douglas City (RM 91.8) by encroachment of riparian vegetation that has occurred since TRD construction.

As these established plants grew, elevated hydraulic roughness generated by the stems and dense understory along the low water channel encouraged fine sediment deposition during tributary-derived high flows, providing seedbeds for additional plants. Their foothold on previously dynamic alluvial bars soon became permanent, such that by 1970 Lewiston releases were incapable of scouring the bars or the trees.

Impact to the mainstem riparian community was more serious than a shift in riparian acreage accounting. Community structure was simplified by a reduction in diversity, with an understory now dominated by dense blackberry. Cottonwood forests, which require overbank deposits and channel migration for initiation and establishment, have disappeared.

Riparian Berm Formation

Deposition of fine sediment within newly encroached riparian plant stands created levee-like features along the low-water's edge, referred to as “riparian berms”. They are now ubiquitous depositional features throughout the mainstem, signaling a change in alluvial behavior riverwide. Riparian berms formed within the historical active channel margin. Low flows released in the late 1960s and early 1970s were well below the flows required to inundate the pre-TRD active channel margin. Willow growth flourished near this low flow waterline, then colonized upslope to the first sharp slope break. This break was at the active channel margin, corresponding to the elevation of pre-TRD high winter baseflows. The varying width of the present-day riparian encroachment band probably reflects, in most locations, pre-TRD active channel dimensions. The progression of riparian colonization onto the Gold Bar median bar (see photos below) illustrates this widening of the riparian zone at the riffle crest where the pre-TRD active channel gently sloped up the median bar. Along the steep bank of this active channel, upstream from the riffle crest, riparian encroachment has been restricted to a relatively narrow band.

Gold Bar (RM 106.3) in 1961, showing exposed cobble/gravel surfaces and patches of riparian vegetation typical of pre-project conditions. Note woody debris on right bank (looking downstream) floodplain.	Gold Bar (RM 106.3) in 1970, showing effects of seven years of riparian encroachment on alluvial deposits. Note thick riparian band developing along low-water surface.
Gold Bar (RM 106.3) in 1975, showing twelve years of riparian encroachment. Note minimal effect of January 1974 14,000 cfs flood on riparian berm.	Gold Bar (RM 106.3) in 1997, showing the current status of morphology downstream to North Fork Trinity River. Note that willow patches on old right bank (looking downstream) floodplain are same trees as shown in 1961 photo.

Today, riparian berms exceeding 7 feet in height are extensive below Junction City (RM 80.0). Some riparian berms are still aggrading but at highly variable rates. The 20-year-old alders in the Sheridan bank-rehabilitation site (RM 82.0) were buried by only 0.8 foot of fine sediment though they were rooted 5 feet high on the riparian berm. In contrast to this slow accretion (at least since the mid- 1970s), recent blackberry understories along the left bank of the Gravel Plant monitoring site (RM 105.5) trapped several feet of coarse sand in one 6,000 cfs dam release in WY1992 (Trinity Restoration Associates, 1993). Riparian berms can continue aggrading if higher flood elevations are experienced, if the riparian berm vegetation becomes even denser, or if fine sediment supply increases.

Special Status Plant Species of the Mainstem Trinity River

In the Trinity River Basin, plant species of special concern fall into four natural ecological groupings based on habitat types and responses to natural and human-caused disturbance:

• Serpentine Endemics
• Rock Dwellers
• Riverine-Riparian
• Forest taxa

A preliminary list of 27 special-status plant species inhabiting or potentially inhabiting the Mainstem of the Trinity River is presented in the table below. These species are known to occur or potentially occur at proposed bridge and restoration sites. Special-status vascular plants include taxa that are:

• designated as Rare, Threatened, or Endangered by state or federal governments;
• proposed for rare, threatened, or endangered status by state or federal governments;
• state or federal Candidate species; and
• species of Concern by the USFWS or California Native Plant Society (CNPS).

Below is a preliminary list of special-status plant species that occur or potentially occur along the Mainstem of the Trinity River and at proposed bridge and restoration sites.

List of Special Status Plant Species