Rush Creek bottomlands: wooded wetlands in the arid Mono Basin
Editors Note: The Rush Creek bottomlands is one of the focus points for restoration in the Mono Basin. The following two pages highlight one very important aspect of the restoration processchannel openings. Here weve brought together scientific and historical background with actions currently underway to illustrate the importance and complexity of channel opening itself as well as the process of restoration itself.
Immediately below the Narrows, Rush Creek widens out dramatically into a broad-bottomed alluvial valley formed over thousands of years by the unique situation of stream processes interacting with a widely fluctuating lake. This interaction resulted in the former V-shaped valley partially filling in with deltaic sediment to form the current broad-bottomed one. Following is a description of Rush Creeks multi-channeled bottomlands prior to diversions. The text is excerpted from Past and Present Geomorphic, Hydrologic, and Vegetative Conditions on Rush Creek by Dr. Scott Stine.
Because of the broad, relatively flat nature of the bottomlands, floodwaters (and with them waterborne seeds of riparian vegetation) could be spread far and wide, naturally irrigating hundreds of acres of lowland. Flows that overtopped the channels carried silts and fine sands and deposited them on the valley floor. The low latitudinal and longitudinal gradients of [the bottomlands], in concert with the narrow, unincised nature of the channels, maintained a high water table.
This combination of factors created an environment in which water was abundant at or immediately below the ground surface. Seepage and overflow water filled oxbows and other depressions: wet meadows were common (roughly 65 acres in the bottomlands); and a high water table underlying deposits of moisture-retaining silts and sands supported a large area (roughly 208 acres) of riparian woodland (mainly of willows and cottonwoods, but including scattered pines and buffaloberry). The wetland nature of the bottomlands was further increased by the naturally and artificially induced seeps and springs that emanated from the canyon walls, and by diversions into Indian Ditch an artificial irrigation canal that carried water to, and distributed it along, the west-central margin of the bottomlands.
In 1929 Rush Creek followed a straight to sinuous, single- to multi-channeled, willow- and cottonwood-lined, low- to very low-gradient waterway across the bottomlands. Aerial photographs, as well as field inspection of the now-abandoned segments of the waterway, show that the channels were narrow (typically 1018 feet wide) and steep-sided. Overhanging walls, rootwads, and deep pits in the channel bottom were common (these persist at several sites within remnant channels). In such channels, even a moderate amount of flow (i.e., ~30 cfs) created relatively deep water (say, 24 feet deep and more depending on channel-bottom intricacies). At these moderate flows, water reached depths exceeding 2 feet along thousands of linear feet of channel through the bottomlands.
From the report: Stine Ph.D., Scott. Past and Present Geomorphic, Hydrologic, and Vegetative Conditions on Rush Creek. Trihey & Associates, 1992.
Channel-openings for 2000 field season to be discussed this spring
Changes on Rush and Lee Vining creeks over the last few years are prompting discussion about the stream channel openings ordered by the State Water Board. Scientists recommendations about these channel openings will be made to the State Water Board next April.
Unplugging the entrances to channels in the stream bottomlands was a key element of the Water Boards order on Mono Basin restoration. Entrances to many of the channels that once threaded the bottomlands in a complex, multiple-channel system (see preceeding article) were plugged during the years of diversions and degradation. Periodic floods scoured out new, straighter channels through the desiccated floodplain and deposited debris in side channels mouths. Today, some of these cut-off channelsthough dryremain reasonably intact in form.
Plans for channel openings were drawn up in 1995 and 1996. High flows in 1997 and 1998 have caused the streams channels to migrate and downcut in various locationsall part of natural stream dynamics, but intensified on the still recovering Mono Basin streams. Today, Dr. William Trush, the scientist in charge of stream monitoring, feels that there are enough changes that the original plans for opening the channels may need to be reviewed.
The overall restoration goal for the Mono Basin streams is to reinstate stream processes and conditions. In both its 1994 decision protecting Mono Lake and the 1998 decision approving restoration activities, the Water Board ordered certain plugged channels to be opened. Other active and passive restoration measures called for by the Water Board include reinstating flows, excluding grazing from the riparian zone, closing roads in the floodplains, and planting willows, cottonwoods, and pines as needed.
Already completed channel openings
A number of channel openings were undertaken on the streams beginning as early as 1991. On Lee Vining Creek, eight channels were opened during 19911994. Since then some channels were cut off from the main stream during the January 1997 flood, and some others were significantly modified in how and when they carry water. The four remaining reopened channels continue to carry water year-round. The scientists monitoring Lee Vining Creek are particularly fascinated by one of the reopened channelsreferred to as the A-4which over the years since it was opened has increased the amount of water it carries and which is developing good meanders and pools with healthy trout.
On Rush Creek, one major bottomland side channelChannel 10was opened in 1995, and three more were opened in the stretch of Rush Creek above Highway 395 in 1999. Channel 10 continues to evolve. Similar to Lee Vinings A-4, Channel 10 has increased the amount of flow that it takes relative to the main channel. Two of the channels opened this fall above Highway 395 on Rush Creek had been mechanically closed at some point in the past. DWP crews used a backhoe to simply pull the cobble dams away from the entrances. These channels are designed to carry water at peak flows. A third channel in the vicinity was opened to carry flows year-round.
Channels remaining to be opened
A number of channels remain to be opened on Rush Creek, all but one of them in the bottomlands below the Narrows. Two proposed openings in particular would rewater large areas of Rush Creeks bottomlands. Today, both these channels are carrying flows at high water, and one carries a minimal flow even during low-flow periods. With some work, they both might carry water year-round.
Bill Trush will be analyzing the situation and making recommendations to the Water Board next April.
The Committee believes that unless compelling new evidence is found, planned channel openings should proceed next year.
Heidi Hopkins is the Committees Eastern Sierra Policy Director. In the winter she enjoys ice skating on ponds in and around the Mono Basin.