The 1997 New Year’s flood in the Walker River Basin, California and Nevada

 

Nathaniel Bergman

CEE 6440

GISWR

 

Introduction

 

The interest in high magnitude low recurrence floods can be generally divided into two main aspects: 1. the human aspect where usually heavy damage is caused to infrastructure, private and public property and occasionally human causalities due to massive inundation and routine life of the affected area is disrupted or completely halted. 2. The scientific aspect where the geomorphic debate of which type of floods shape the environment and are more important for landscape evolution: small and medium floods that are common and do most of the work (Wolman and Miller, 1960) or high magnitude low recurrence floods that strike once every few decades or centuries and cause mass devastation (Baker, 1977, Wolman and Miller, 1978). High magnitude low recurrence floods such as the Missoula floods channeled through the Columbia River during the last ice age in eastern Washington left behind impressive erosional and depositional features (Baker, 1973) while the more recent Great Flood of 1993 in the Missouri and Mississippi Rivers that flooded 9 states and killed 47 people left behind little geomorphic change or clues of its occurrence (Gomez et al., 1995). Large floods are not only scientifically important but are also very important to long term infrastructure design (e.g. bridges, roads, culverts and other infrastructure next to water) and therefore documenting their behavior is crucial to engineering decision-making and consequently monetary costs to society (Baker et al, 2002).        

 

During the new year of 1997 a heavy rain on snow event melted the Sierra Nevada snow cover and generated rare high magnitude floods with low recurrence intervals in Oregon, California and western Nevada. In the eastern Sierra, the floods caused major inundation and heavy damages especially in three basins of eastern California and western Nevada: the Truckee River through Reno into Pyramid Lake, the Carson River through Carson City into the Carson Sink and the Walker River through Antelope, Smith and Mason Valleys and their major city of Yerington towards Walker Lake. While the first two floods are well documented (for example Hoffman and Taylor, 1998; Carroll et al., 2004) mainly due to the interest in heavy mercury contamination, large cities along the flow paths (such as Reno, Sparks and Carson City) and infrastructure damage, the Walker River flood of 1997 is only a record number in the USGS hydrological files of various gauging stations along the upper basin and by the local residents. The flood attenuated downstream suggesting that transmission losses or overbank flow was substantial and when reaching the lower gauging stations of the Walker River the flood is no longer the largest on record. Surprisingly, an initial reconnaissance of the basin during August 2009 did not reveal any traces of the flood in Mason Valley (e.g. flood deposits or massive erosion) or we did not detect channel migration or avulsion using pre and post flood aerial photographs. While this outcome is expected in areas where a recovery efforts took place (especially localities where homes, fields and infrastructure were affected), natural areas ordinarily contain some geomorphic signature of channel change. This is especially true for aridic regions that usually preserve fluvial stratigraphic features much better than humid climate regions. The second fieldwork session conducted in October-November discovered that the mass of sediment that the 1997 flood scoured from the Walker Canyon and imported from the Sierra still lies upstream above Topaz Lake reservoir in the form of massive bars that only high discharges can entrain downstream. This work’s part of my PhD – we were invited to portray the Walker River sedimentary regime in Mason Valley Nevada and then move upstream towards the California headwaters. We found out that a substantial change occurred during and after the 1997 flood and therefore we need to understand what happened during that flood as a starting point for our work. This work will be my first attempt in that direction and further work (field and lab) will be carried out as a result in the near future.     

 

Walker River Basin study area

 

The closed Walker River Basin drains part of the eastern Sierra Nevada in California by two main tributaries that flow in a northeasterly direction: the West Fork and East Fork Walker Rivers that converge in Mason Valley, Nevada, just south of the city of Yerington. From there, the river flows northeast to Campbell Valley and then near Wabuska u turns southeast into the terminus Walker Lake, a small remnant of pluvial Lake Lahontan (Figure 1). The basin’s area extends over 6480 km2 (4050 mi2) but once exiting the mountainous part of California that belongs to the Pacific Mountain System physiographic region into Nevada both tributaries are flowing within a high desert terrain (rain shadow effect of the Sierra mountains producing cold winters and hot summers) typical to the Basin and Range province on the western part of the Great Basin. Elevations range from 3760 m at the Sierra headwaters to 1200 m at Walker Lake. Average annual precipitation at Yerington is 135 mm (during 1971-2000).

 

Geology of the Walker basin is diverse consisting of consolidated rocks that range in age from Triassic to Quaternary and primarily consist of quartz monzonite, granodiorite, basalt, rhyolite, and andesite. The Singatse and Wassuk Ranges are Cenozoic fault-block structures, indicating continuity with the Sierra Nevada (Moore and Archbold, 1969). Basin-fill deposits are unconsolidated Tertiary and Quaternary alluvial sediments that underlie the valley floors and alluvial fans near the base of mountain ranges. Valley-floor sediments are fine sand, silt, and clay alluvium and playa clay and sand, and fan sediments are primarily gravel, coarse sand, and silt with some talus material (Huxel, 1969). Lacustrine clay deposits at least 200-feet thick are exposed near Weber Reservoir and may extend from Walker Lake to the high-water stage of Pleistocene Lake Lahontan in Mason Valley as far south as Yerington (Moore and Archbold, 1969, Reheis, 1999). Soils in the basin are Aridosols and Mollisols. Vegetation consists of Aspen-fir and Sonoran sagebrush on high elevations, juniper, and sagebrush grasslands on the lower areas. The two tributaries gradually turn from coarse gravel-bedded rivers into sand-bedded rivers (downstream fining) in the valleys although there are gravel recruitment points along the flow path (such as from perennial and ephemeral tributaries that occasionally produce flash-floods during summer thunderstorms, exposed desert hillslopes with sparse vegetation, cattle trampling of the banks that erode sediment into the channel and narrow reaches such as Hoye and Wilson Canyons that are rich with loose sediment available for transport).

 

 

 

 

The two tributaries and the main stem Walker River support a vast agricultural system in the lowland valleys that includes various crops (such as alfalfa and onions) and large scale cattle grazing. Due to the high demand for water in this arid region, the two tributaries and the main stem of the Walker River maintain an altered hydrograph that does not resemble a snowmelt flood regime typical to this area but rather a reduced multi-peaked prolonged hydrograph designed to sustain the irrigation and grazing needs of the local ranchers throughout the growing season. This is achieved by means of two large reservoirs: Topaz Lake on the Nevada-California borderline located on the West Walker River in Antelope Valley and Bridgeport Reservoir located near the city of Bridgeport California. These two reservoirs are filled during the spring freshet in order to allow irrigation during the dry seasons of summer and early fall. Mason Valley has an extensive canal system that feeds the ranches and their fields along the valley and the excess drainage flows through ditches back into the river. Another storage reservoir – Weber Reservoir is located downstream on the main stem Walker within the Walker River Paiute Indian Reservation. The average water contribution to the main stem between the East Walker and the West Walker is 0.56 and 0.44, respectively (based on USGS data from 1939 to 1993). Most of the water (54%) the two tributaries carry into the main stem Walker is lost within Mason Valley to irrigation, evaporation, transmission losses into groundwater and evapotranspiration through riparian vegetation along the river corridor.  

 

Methods

  • Obtain weather data from NOAA website.
  • Acquire data from the USGS website.
  • Obtain LiDAR of the basin from the Bureau of Reclamation.
  • Data collected and published by the Nevada Bureau of Mines and Geology
  • Evidence of witnesses present during the 1997 flood.  

 

 

 

The 1997 New Year’s record flood

A cool winter storm brought valley rain and several feet of mountain snow to southwest Oregon, northern California, and western Nevada on December 21 and 22, 1996. This system only set the stage for what would become one of the most historical flood events to affect the region in recent times. Beginning Christmas Eve, the overall weather pattern began to shift from a polar air mass toward a warmer and wetter tropical regime with promises of a noteworthy precipitation event (Figure 2). Not only would the relentless precipitation through early January 1997 bring widespread flooding, but the snow pack from the pre-Christmas storm would significantly melt to only exacerbate problems from the excessive amounts of runoff. Major highways, including Interstate 80, and US Highway 50 were all closed at some point during the event.

 

The following meteorological information will explain the general synoptic scale evolution of the event, while the hydrological data will show the results of the excessive amounts of precipitation and contribution from the melting snow pack. This data will put the scope of the widespread flooding into perspective. A cool winter storm brought valley rain and several feet of mountain snow to southwest Oregon, northern California, and on December 21 and 22, 1996. This system only set the stage for what would become one of the most historical flood events to affect the region in recent times. O’hara et al (2007) described the 1997 flood as “The major weather-related event in the region’s history”. Beginning Christmas Eve, the overall weather pattern began to shift from a polar air mass toward a warmer and wetter tropical regime with promises of a noteworthy precipitation event. This is presented in the satellite image in Figure 2. Not only would the relentless precipitation through early January 1997 bring widespread flooding but the snow pack from the pre-Christmas storm would significantly melt to only exacerbate problems from the excessive amounts of runoff.

 

A shift in the weather pattern brought warm storms of tropical origin across the region from December 26, 1996 through January 3, 1997, with the most potent system affecting the region at the turn of the year. This change occurred after a cool winter storm affected the region just before Christmas on December 21 and 22, 1996. This polar system left behind several feet of snow over the mountainous terrain; a snow pack that would contribute to the flooding issues just over a week later. With the tropical air mass storms, precipitation fell across much of the west coast with a focus of excessive precipitation over the higher terrain from western Washington southward to northern California and western Nevada. The Oregon Climate Services created a graphical re-analysis of the precipitation for the period of December 29, 1996 through January 3, 1997, which indicated the widespread nature of these storms (Figure 3). As expected, precipitation totals at lower elevations were dramatically less. Ideal mid-level flow resulted in a significant contribution from orographic enhancement on south and west facing slopes. Climatological precipitation ratios between the west slopes of the Sierra Nevada and the adjacent Sacramento Valley are near 3:1. During the period from December 26, 1996 through January 3, 1997, observed ratios were approximately 10:1. These observed ratios show the extent of the excellent orographic enhancement of the precipitation. Although less than their counterparts on the west slope of the Sierra Nevada, significant amounts of precipitation along with a melting snow pack contributed to the widespread flooding across the eastern Sierra Nevada and portions of western Nevada. The greatest amounts were recorded near the crest of the Sierra Nevada (in excess of 20 inches) with lesser amounts over points to the east. Table 1 below indicates precipitation totals at selected locations for the 9-day period ending January 3, 1997. Although the table summarizes only one location in the southern Walker Basin (Bridgeport with a mere 2 inches), the West Walker River higher elevations are probably close to Tahoe City, Truckee and Markleeville while the lower elevations probably resemble Carson City. Therefore, I assume the precipitation over the Walker River Basin ranged from 2 to 18 inches (NOAA website).

 

 

 

Table 1. Eastern Sierra Nevada and Western Nevada 9-Day Precipitation Totals
ending January 3rd 1997 in inches (Source: NOAA website). .

Location

River Basin

9-Day Totals
12/26/96-01/03/97

 Susanville

 Susan

5.51

 Tahoe City

 Truckee

13.63

 Truckee

 Truckee

12.74

 Reno-Tahoe Int'l Airport

 Truckee

2.15

 Markleeville

 Carson

8.13

 Carson City

 Carson

4.27

 Bridgeport

 Walker

2.16

 

 

Across the eastern Sierra Nevada and portions of western Nevada, the series of storms caused an estimated $1 billion in damages. Numerous homes and businesses suffered extensive damage, while the transportation system experienced widespread issues. Major highways were closed due to the flooding or subsequent mudslides, including Interstate 80 both east and west of Reno, US Highway 395 to the north and south of Reno, and the Mount Rose Highway (Nevada State Highway 431). Flood waters from the West Walker River destroyed several miles of Highway 395 through the narrow Walker River canyon. In Wilson Canyon also on the West Walker River, the State Road 208 was damaged by the floodwater and transportation was completely cut between Smith and Mason Valleys (Figure 4).   

 

 

 

 

In order to investigate the extent of flooding the above rain and snow produced, I extracted from the USGS websites flood hydrographs of the 1997 flood. I initially chose three gauging stations: West Walker River near Hudson, East Walker River above Strosnider Ditch and Walker River main stem at Wabuska. The reasoning behind this choice is the fact that these gauging stations are below the 2 reservoirs (Topaz Lake and Bridgeport Reservoir) and allow to see the attenuation effect of the 1997 flood without any further human intervention. The two gauging stations are located at the entrance into Mason Valley (East Walker and West Walker) and its outlet (Walker main stem after they converge in the valley south of Yerington). It has to be acknowledged that Topaz Lake is a diversion reservoir and even the fact the diversion was completely open to extract as much water as possible from the West Walker the attenuation was limited (the peak discharge at Colleville California before the reservoir was 12,500 cfs compared to 11,400 cfs downstream near Hudson Nevada below Topaz Lake assuming water contribution from tributaries is minor) while the Bridgeport Reservoir is situated on the main stem of the East Walker and its dam’s closure was successful at attenuating most of the flood into a usual magnitude flood event. Figure 5 presents the mean daily flow hydrographs of the 3 gauging stations starting December 27, 1996 till March 30 1997. It is eye-catching that even daily mean flow hydrographs show the extensive attenuation along the 20 miles segment of Mason Valley. Looking at the peaks of the 3 rivers that year shows that the East Walker input and Wabuska output are identical (2610 and 2600 cfs, respectively) while the West Walker input peak is a magnitude higher at 11,400 cfs (Figure 6). Just to get a sense of the magnitude of the 1997 flood the mean annual flow of the West Walker is 210 cfs, the East Fork has a 164 cfs mean annual flow and at Wabuska outlet the mean annual discharge is 165 cfs.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It’s obvious that the East Walker and the West Walker did not peak at exactly the same time but it is tempting to say that the entire peak of the West Walker did not exit the valley. To explore this hypothesis a more precise data set is needed thus I acquired the instantaneous data of the 1997 flood from a different server of the USGS (off-line) to see the real peak flows and not mean daily flows. Unlike the previous data that the website allows you to build graphs or obtain the data in various forms, the dormant server only gives you limited options and the graphs have to built manually using Excel. Figure 7 presents the instantaneous data for the 3 gauging stations. Although the time has to be aligned (x-axis), as speculated, the entire peak of the West Walker River was truncated at Mason Valley. This explains the wide range of overbank flooding in the city of Yerington. Residents of Yerington and the valley that I interviewed claim that the overbank flooding of the Walker main stem is partially a result of poor management of the river, building too close to the river banks on the active floodplain and that several structures such as the airport served as obstruction to the flow. Figure 8 obtained from the Nevada Bureau of Mines and Geology shows the extent of flooding within Mason Valley (Rigby et al., 1998). It can be seen that there’s a note on the figure that near Yerington Mine the road was broken deliberately to allow the water to flow freely downstream. These claims will be checked and verified next year when the fieldwork resumes and once we model the flood through the valley.       

 

 

 

 

Water volumes calculated from the hydrographs presented in Figure 7 basically show the same theme during 9 days of rain: the East Walker brought 5,098 acre/foot of water, the West Walker brought 22,612 acre/foot of water and the main stem Walker exported only 5,157 acre/foot of water that almost resembles the East Walker input. In order to see the reduction of the flood peaks I looked at 3 more gauging stations: West Walker near Coleville California (this is the most upstream active gauging station of the USGS), West Walker at Hoye Bridge below Topaz Lake reservoir and Walker River (main stem) at Schurz downstream of Weber Reservoir not too far from Walker Lake. The dramatic reduction in peak flow occurs once the river leaves the gravelly reaches and enters sand-bedded reaches. Figure 9 presents the reduction in peak discharges and concurrent decrease in flood wave velocity downstream.

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flood frequency analysis for Mason Valley reveals that using the modern record of the 3 USGS gauging stations (less than 100 years) is not able to portray the 1997 flood in a precise way. Figure 10 shows that all three stations flatten at some point around 2000-3000 cfs (upper bound?) but in the West Walker there’s a spike towards the peak of the 1997 flood that looks somewhat artificial and not in place. This suggests that a different statistical approach is needed such as the one used in paleoflood hydrology. A work done by Mann (2000) that analyzed the paleoflood hydrology of the West Walker River in the upper reaches just below the Sierra Nevada of California estimated the 1997 flood at 400-450 year flood (recurrence interval of 0.0025-0.0022). Even with intense attenuation due to overbank flow and transmission losses to the channel bed it is doubtful the flood in Mason Valley entrance was less than a 100 year flood event (recurrence interval of 0.01).    

 

 

 

In order to model the flood in the 3 valleys (Antelope, Smith and Mason) once the tributaries leave the high Sierras, the LiDAR dataset from the Bureau of Reclamation was obtained (Fig. 11). This allowed extraction of x-sections and long profile and the idea was to then to model the flood using a hydraulic program such as HEC-RAS. Unfortunately, it was found to be unproductive as LiDAR does not penetrate water (green LiDAR does but this was not utilized in the Walker) and needs substantial filtering and correction of the raw data before it can be used (only the floodplain was found to be very precise). At first the long profile showed many undulations (Figure 12 top) that are not typical to a river thalweg while the x-sections extracted using the 3D Analyst tool had a triangular, gully-like shape, that is not a characteristic to the trapezoidal or rectangular x-sections of the Walker valleys (Figure 12 bottom). This needs much work before utilization of actual modeling of the 1997 flood.     

  

 

 

 

 

 

Conclusions (preliminary)

The unusual precipitation amounts of the 1997 New Year from an unusual tropical air mass and antecedent snow storms that produced a high snow pack a week before created unusual atmospheric conditions in the west coast. The floods that followed in Oregon, California and Washington were well documented especially for rivers passing through large cities or devastating infrastructure but for the Walker River Basin in the eastern Sierra Nevada California and western Nevada the data has to be collected from numerous agencies and the flood story has to be built almost from scratch. The flood has left the Sierra as a record flood and once it reached downstream lost most of its volume until it is no longer the flood of record for the downstream gauging stations. Collection of data from various agencies has to be combined in order to explain the sedimentary change the Walker River has undergone since the flood of 1997 and field evidence support that 13 years later the legacy of this flood still persists in the upper valley (Antelope) and probably propagates downstream slowly into the other 2 valleys (Smith and Mason) as a function of the annual flow competence. Using GIS tools such as the LiDAR data once it is calibrated with a GPS survey and precise mapping of river units (especially of storage zones such as giant point bars) will be a valuable tool in estimating the amounts of sediments that are currently within the fluvial system and what are the measures needed to be taken in order to manage the river in a proper way it will continue to be the lifeblood of the region but will not become a geo-hazard once another flood of the New Year’s kind returns.      

 

Further work derived from this work

  • LiDAR needs calibration with GPS surveying before any modeling is done.
  • Once the x-sections can be extracted from the LiDAR run HEC-RAS first only with water to see their behavior as they move downstream and once it achieves results that are calibrated with the various USGS gauging stations run it again with inherent sediment transport.
  • Obtain the groundwater response to the flood so the transmission losses and overbank flow can be assessed correctly.
  • Obtain the snow data from SNOTEL and California Snow Survey (NRCS) to actually find the rainfall-runoff ratio of this flood event.
  • Map storage zones and asses the amount of sediment that is currently in the system.
  • Obtain data from CALTRANS and Nevada Department of transportation regarding their bi-annual inspection of bridge scour within the Walker Basin.
  • Acquire all the documents regarding the heavy mechanic equipment work done by the US Army Corps of Engineers following the flood to re-regulate the river.

 

References

 

Baker, V.R., 1973. Paleohydrology and sedimentology of Lake Missoula flooding in Eastern Washington. Geological Society of America Special Paper 144, pp. 1-79.

 

Baker, V.R., 1977. Stream channel response to floods with examples from central Texas. Geological Society of America Bulletin 88, pp. 1057–1071.

 

Baker, V.R., Webb, R.H. & House, P.K. The scientific and societal value of paleoflood hydrology. In: House, P.K. Webb, R.H., Baker, V.R., Levish, D.R. (Eds.) Ancient Floods, Modern Hazards: Principles and Applications of Paleoflood Hydrology. Water Resources Monograph 5, AGU, Washington, D.C., 2002, pp. 1-19.

 

Gomez, B. Mertes, L.A.K. Phillips, J.D. Magilligan, F.J. James, L.A. 1995. Sediment characteristics of an extreme flood: 1993 upper Mississippi River valley. Geology 23, pp. 963–966.

 

Mann, M.P. 2000. Use of geomorphic information in extending the flood record of the West Walker River, California. Unpublished Master Thesis, University of Nevada, Reno, 142 p.

 

Moore, J.G., Archbold, N.L., 1969, Geology and mineral deposits of Lyon, Douglas, and Ormsby Counties, Nevada. Nevada Bureau of Mines Bulletin 75, 42 p.

 

NOAA website: http://www.cnrfc.noaa.gov/storm_summaries/jan1997storms.php. Last accessed on November 26, 2009.

 

O’hara,  B.F. Barbato, G.E. James, J.W. Angeloff, H.A. Cylke, T. 2007. Weather and Climate of the Reno-Carson City-Lake Tahoe Region. Nevada Bureau of Mines and Geology Special Publication 34, p. 71. 

 

Reheis, M., 1999. Extent of Pleistocene lakes in the western Great Basin. U.S. Geological Survey Miscellaneous Field Studies Map MF–2323.

 

Rigby, J. Crompton, E.J. Berry, K.A. Yildirim, U. Hickman, S.F. Davis, D.A. 1998. The 1997 New Year's floods in western Nevada.
Nevada Bureau of Mines Special Publication SP23, 111 p.

USGS websites: http://nevada.usgs.gov/walker/ about Walker River and Lake and http://waterdata.usgs.gov/nv/nwis/current/?type=flow USGS Real-Time Water Data for Nevada, National Water Information System. Last accessed on November 26, 2009.

 

Wolman, M.G., Gerson, R., 1978. Relative scales of time and effectiveness of climate in watershed geomorphology. Earth Surface Processes and Landforms 3, pp. 189–208.

 

Wolman, M.G., Miller, J.P., 1960. Magnitude and frequency of forces in geomorphic processes. Journal of Geology 68, pp. 54–74.