Needing to burn a few vacation days, I set my sights on visiting the Hoover Dam. I had never been there in all my travels, and ever since visiting the Garrison Dam in North Dakota years ago, I have been fascinated by these massive structures. It's an engineering marvel, and I was interested in learning its history firsthand. Of course, any trip I plan these days is guaranteed to include hunting for survey marks and hiking at least one local trail.
I started my trip early on Sunday morning and planned to stop along the way looking for roadside survey marks; with no pressing schedule, I turned a five-and-a-half-hour drive into a full day of perpetual stops to look for survey marks, netting eleven recoveries for the day.
Yesterday was a planned hiking day, and I had chosen a challenging trail, the Gold Strike Hot Springs Trail, an almost four-mile out-and-back route through Gold Strike Canyon. There were many dry falls and large boulders to navigate, many of which had permanent fixtures for climbing ropes to help get through the challenging areas. My primary goal was to find the hot springs and spend plenty of time soaking! The trip did not disappoint; the hike was a great workout, the scenery was incredible, and the hot spring was absolutely worth the work!
Robert L. Mendenhall Scenic Overlook
On my way to the trailhead yesterday, I zipped past this Scenic Overlook. As I passed, I noticed a granite obelisk and immediately wondered if it was a survey monument of some sort 🙄 (yeah, that's how my mind works!🤣). I wasn't too worried as I knew I was coming back this way today on my way to the dam.
I decided to make the viewpoint the first official stop of my 'Hoover Dam Day,' and I'm glad I did! The Scenic Overlook was named for Robert Mendenhall, the founder and president of Las Vegas Paving Corporation. He invented and patented the Asphalt Recycling Process and numerous other inventions that benefit the construction industry and help conserve our natural resources. Bob was inducted into the Nevada Inventors Hall of Fame in 1984. He passed away at the age of 90 on June 2, 2018.
The Obelisk
While not a survey monument, the obelisk is very cool and includes excellent information about the dam. The dam remains the highest concrete dam in the United States and, at its completion, the largest in the world (today, it ranks as the 18th largest in the world).
In addition to regulating river flow and providing flood control, the dam stores water and supplies it to Arizona, California, Nevada, and Mexico. The dam generates enormous electricity due to the height difference between the reservoir at the top and the downstream outlet.
In this area, everything is about the dam! As I walked to the end of the overlook to take a picture of Lake Mead, I was blown away by this incredible mosaic of the dam.
After spending some more time reading all the interpretive signs, I hopped in the Jeep and headed toward the Mike O'Callaghan - Pat Tillman Memorial Bridge.
Mike O'Callaghan - Pat Tillman Memorial Bridge
This was my first trip to the Hoover Dam and the surrounding area, and my preliminary research focused more on dam history, survey marks, and hiking trails. I visited the bridge mainly to get good pictures of the dam; little did I know it had its own story! I pulled into the parking lot at 7:30 a.m. There were already three tour buses in the lot and a line of people snaking up the switchbacks of the sidewalk toward the pedestrian walkway.
In retrospect, I should have taken a photo of that scene because it was a healthy walk. Most people stayed on the ramps that formed switchbacks up the side of the hill. Taking the stairs offered a direct route that was less crowded and comparatively more work, making it an excellent workout first thing in the morning!
Many interpretive signs along the way described the bridge's history and the area's geology; I spent some time taking in the information before setting out across the bridge. As expected, a large group of people in the middle of the bridge were taking photos; I passed through the crowd and opted to take my pictures from a little further down the pedestrian walk to avoid the crowd. Much to my surprise, a few hundred feet past the "selfie spot," I found these two commemorative bronze plates in the walkway!
The first one I came to marked the Nevada - Arizona State line, definitely worthy of being added to my survey mark database as a commemorative mark. 👍🏻 A few feet away was the second plate commemorating the arch's highest point! These plates were approximately two feet in diameter; I was surprised how many people on those tours never saw them because they stopped for selfies before reaching them, then turned around and returned to the bus. I only saw three or four people walking to the far end of the pedestrian walkway (the Arizona side is blocked off.)
Bridge Informational Plaques
Large bronze interpretive signs detailing facts about the bridge were placed at regular intervals on the walkway. On my way back across the bridge, I photographed each sign and arranged them in the photo carousel below. I've added a "Guide" photo showing each sign's position on the bridge; this was fascinating information!
For better viewing, click on the image to enlarge the view and use the navigational arrows < > on the left and right sides of the photos to cycle through them.
This was a pleasant surprise on my trip. In addition to the fantastic views, there was lots of excellent information. If you'd like to learn more about the bridge, I encourage you to check out the Department of Transportation Federal Highway Administration article, "A Majestic Showcase."
The Survey MArk Scavenger Hunt Begins - It's SMASH Time!
Arizona
Recovery Date: October 29, 2024
Total Marks Recovered: 10
Unable to Locate: 2 (No Access)
Confirmed Lost/Destroyed: 1
Discs Recovered: 9
Non-Disc Type Recovered: 1 (Hex Nut)
After visiting the bridge, I continued on the Hoover Dam Bypass and took the first Arizona Exit, Kingman Wash Access Road, for the first recovery on my list, Vertical Control Mark L 485 (PID: GR1328) - this National Geodetic Survey (NGS) disc was located in a granite rock outcrop on the side of Old US 93. Unfortunately, the mark was located within a one-mile section of Old US 93 that is permanently closed, gated, and marked "No Trespassing U.S. Government Property." 🤷🏻♂️
I've made many recoveries on abandoned roads gated and closed to vehicle traffic. Still, given the security around the dam, I just marked this one as ⛔️ (No Access) and made my way back to Old US 93, crossed over the dam, and headed to the easternmost Hoover Dama Parking Lot on the Arizona side, Lot 16, to look for my next mark.
BENCHMARK J 134 (PID: GR0601)
This standard U.S. Coast & Geodetic Survey (USC&GS) Benchmark was in a rock outcropping near Parking Lot 16 and was easy to find due to the witness sign next to it. It was still early (9 a.m.), and no one was parked in the lot yet.
However, while I was making this recovery and taking my photos, a Bureau of Reclamation Security Response Force (SRF) K-9 Unit rolled up in the parking lot and hung out there until I walked back to the Jeep. 🤔
Parking Lot 15 - Hoover Dam Lookout and Gift Shop
The next mark on my list was USC&GS Benchmark H 134 (PID: GR0597), set in a rock outcrop near the parking area. Like J 134, this should be within 25-30 feet of the sidewalk around the parking area; however, the rocks surrounding the parking lot were heavily signed "No Climbing" and "Stay Off." I didn't see any witness sign, so I marked this one as "No Access" ⛔️ as well. The SRF Officer was in the parking lot walking around the perimeter with his service dog; if I had seen a witness sign, I would have asked him if I could make a recovery, but I didn't think he'd let me randomly climb around on the rocks looking for the mark. 🤷🏻♂️
This stop wasn't a complete bust; besides great views of the Dam, a lasting piece of WWII history remains on the hillside above the parking lot.
Initially constructed in December 1941 (prompted by the attack on Pearl Harbor), four pillboxes were quickly built with concrete and camouflaged with volcanic rocks mined from the same ridge; each was armed with a machine gun and was manned around the clock.
As noted in the informational sign, three original pillboxes had been dismantled. In 2012, the Bureau of Reclamation and National Park Service stabilized and rehabilitated the remaining pillbox.
Park and Walk
After a quick trip through the gift shop, I hopped back in the Jeep and drove down to Parking Lot 11, the last free parking lot, as I got closer to the Dam. I decided to park there and walk the Dam to recover the rest of the survey marks on my list. There aren't any "bad" places to view the Dam; the two pictures below were taken from the viewpoint in Lot 11.
The four upstream intake towers control water flow into the Dam to generate electricity. They direct water from the reservoir to large steel pipes (penstocks) that carry it to the turbines. The four 395-foot towers are reinforced concrete; two are on the Arizona side, and two are on the Nevada side. The photos above show the high water mark (from mineral deposits) when Lake Mead reached its highest point in 1983, 1,225.44 feet. The image below was taken by a Bureau of Reclamation (BOR) photographer in 1935, pretty much from the same point where I took my pictures. According to our BOR tour guide, the lake is about 100 feet lower than they'd like.
It's easy to get carried away in all the cool facts about the Dam; if you're into all the statistics, facts, and figures, check out the link below:
A stairway led from Lot 11 down to Lot 10 (the only paid parking facility on the Arizona side). My next survey mark target was on the northwest side of the parking lot, on the sidewalk overlooking the Arizona Spillway.
The only time water flowed through the spillways was in 1983. When Lake Mead is full, the water travels around the dam via the spillways. It is channeled through the massive tunnels originally built to divert the Colorado River during the dam's construction. According to the BOR FAQ, if the spillways were in full use, the flow over each would be the same as the flow over Niagra Falls, and the drop from the top of the gates to the river below would be almost three times greater! 😳
BENCHMARK W 135 (PID: GR0595)
After taking the picture above, I walked about 30 feet to my left and easily spotted my next recovery, another USC&GS Benchmark. Easy peasy! 😉
You will note that the survey marks I found on the Dam today are dated 1935, the year the Dam was completed. I found several different types of markers: Benchmarks, Vertical Control Marks, Triangulation Stations, and their Reference Marks. "Benchmark" is a generic term used for any reference point when measuring elevation; Vertical Control Marks are generally more precise than benchmarks and are part of a controlled network. Triangulation Stations and their Reference Marks are horizontal position markers (marking exact latitude and longitude); Triangulation Stations typically have one or more Reference Marks. If the station is compromised or damaged, the reference marks can be used to locate the exact position of the station.
Survey marks serve many purposes; in the introduction to the National Oceanic and Atmospheric Association manual on Geodetic Benchmarks, Benchmarks are defined as "long-lasting points for which elevations have been determined, used to control other surveys and to monitor the movement of and within the Earth's crust." Click on the link to view the complete manual 👉🏻 [NOAA Manual NOS NGS 1] One of the primary purposes of the survey marks established on the dam was to measure the deformation of the earth's crust caused by impounding the water of Lake Mead. If you're a science buff, you will want to check out The First Fourteen Years of Lake Mead, a geological report completed after a comprehensive survey conducted in 1948-49.
BENCHMARK V 135 (PID: GR0594)
The previous Benchmark (W 135) was at the ENE end of the Arizona Spillway, and V 135 was at the WSW end. Click on the image to the right to enlarge it and see their relative positions.
This was my first Bureau of Reclamation Benchmark disc found today. The design is similar to the original National Geodetic Survey Benchmark discs, with the large cross in the center and the text ABOVE, SEA, DATUM, and FEET prestamped on the face.
A datum is a reference point or surface used to define the location of points on the Earth's surface; many different types of datums are used in surveying; the current ones used in the United States are NAD 83 and NAVD88. The North American Datum of 1983 (NAD 83) is a geometric datum that uses an ellipsoid model of the Earth to measure positions. The North American Vertical Datum of 1988 (NAVD 88) measures land elevations and water depths; its origin point is in Quebec, Canada, and applies to the contiguous United States (CONUS).
The cool thing (to me) about the remaining recoveries on the dam is that they are all in plain sight. As I was making my recovery, people would stop and watch me prepare the marks for photos and would ask what I was doing. Of course, I'd give my "elevator pitch" version of what a survey mark is and its purpose. To a person, people who stopped were fascinated and usually took pictures of the mark after I was finished. 😊
Downstream Marks
I went back and forth across the road to make my recoveries; however, I grouped them by which side of the dam they were on. The labels "upstream" and "downstream" are frequently used when identifying various dam features, so organizing these galleries similarly makes sense.
BENCHMARK T 135 RESET (PID: GR0593)
This Benchmark disc was reset by the BOR in 2020, but it still bears the stamped date of 1935, when the original mark was established. The nice thing about the marks set close to the wall or other obstructions is that they are not worn smoothly due to years of foot traffic.
REFERENCE MARK (UNIDENTIFIED)
I spent a lot of time trying to sort this one out. It's in the middle of the sidewalk, so any hope of reading the original stamping is gone; however, it is clearly a Reference Mark. I created the last image to show its relative position to the ARIZONA BASE and HOOVER DAM BLOCK A Triangulation Stations. However, neither one of the datasheets for those marks mentions a reference mark. Of course, the reference mark could have been added later, but I would have expected the datasheet to have been updated.
While researching the marks on the dam, I found a Triangulation Station named "C" on the NGS Map. According to the datasheets (yes, it has two datasheets with different Permanent ID numbers! 🙄), the coordinates place the survey mark "C" on the dam's upstream side in front of the walkway to the Arizona Towers. Both datasheets show the mark as "found," and the coordinates were verified with a Handheld GPS; the datasheets also included photos of the mark—THIS mark! All that means is that a Geocacher found and reported the mark, believing it was the Triangulation Station C.
You can view the datasheets by clicking on these links (GR0588 and GR1849); once on the datasheet, there is a hyperlink to photographs; when you follow those, you'll see pictures of this unidentified reference mark. The fact that both datasheets identified the Triangulation Station as being on the East or Upstream side of the dam should have been enough to prove that this reference mark was not the disc they thought it was. That, and the fact that this mark is a REFERENCE MARK! 😂
TRIANGULATION STATION ARIZONA BASE (PID: GR0591)
I was thrilled to recover the ARIZONA BASE and NEVADA BASE Triangulation Stations. 😁
BENCHMARK R 135 (PID: GR0597)
Again, being closer to the sidewalk's edge, the stamping on this Benchmark is still readable.
Upstream Marks
TRIANGULATION STATION HOOVER DAM BLOCK A (PID: GR0592)
This is another example of how a disc in the primary walkway will wear to a smooth finish, obliterating some stamped data. According to the datasheet, this disc was stamped with the letter "A" above the triangle symbol and the date 1935 below. The station description (often referred to as the "to reach" description) in the datasheet and coordinates confirm that this is station HOOVER DAM BLOCK A.
BENCHMARK S 135 (PID: GR0590)
This is another good recovery in the 135 series.
TRIANGULATION STATION C (PIDS: GR0588 & GR1849) (NOT FOUND, LIKELY DESTROYED)
As mentioned above, Triangulation Station C has two datasheets; both provide coordinates and a description that places the mark as shown in the first image. The only mark I could find anywhere near the supposed location of Station C was a bronze disc at the sidewalk's centerline and roadway leading to the Arizona Tower. The disc is held in place with a mag nail and is unmarked.
I'm including the two pictures to show the location of that unnamed mark and that no other marks are nearby. If I had to guess, the Station C disc was destroyed when the handicap ramps were added to the sidewalk.
CP 3 (PID: N/A)
This Control Point was a "Surprise Proximity" recovery, set at the top of the wall on the dam's upstream side. I didn't check to see if this nut was screwed onto a bolt sunk into the concrete wall or if it was just epoxied into a shallow hole chiseled out of the concrete; either way, it was one of two nuts that I found labeled as "CP" (CP - 1 is on the Nevada upstream side).
Nevada
Recovery Date: October 29, 2024
Total Marks Recovered: 10
Unable to Locate: 1 (No Access)
Confirmed Lost/Destroyed: 0
Discs Recovered: 9
Non-Disc Type Recovered: 1 (Hex Nut)
Crossing the State Line
Continuing on the sidewalk past CP 3, I encountered my first Benchmark on the Nevada side of the dam. I created waypoints in my GAIA GPS App for the ten survey marks with Permanent IDs in the NGS database. As luck would have it, I found CP 1 (my second surprise proximity recovery). I had to mark one Benchmark (C 174, PID: GR0575) as "No Access" as it was located on the plaza undergoing renovations, so I still ended up with ten recoveries.
BENCHMARK Q 135 (PID: GR0586)
It was a little amusing to find this one first because the subsequent recovery was placed on the sidewalk beneath a monument added in 1955 that indicated where the state line was. When the state line is a flowing river, you might be granted some wiggle room for nailing down the exact state line, but these were all engineers! 🤣
TRIANGULATION STATION B (PID: GR0585)
Even the prestamped triangle in the center of this disc is beginning to disappear due to the constant foot traffic. The upside to these marks is they retain their brassy shine! ✨👍🏻🙃
BENCHMARK H 174 (PID: GR0584)
BENCHMARK G 174 (PID: GR0583)
TRIANGULATION STATION A (PID: GR0581)
CP 1 (PID: N/A)
This is CP 1, another surprise proximity recovery; if I had intentionally looked for these, I likely would have found CP 2 and maybe others. 🤷🏻♂️
BENCHMARK F 174 (PID: GR0580)
TRIANGULATION STATION NEVADA BASE (PID: GR0578)
This is the companion disc to the Arizona Base; it shows a little more wear but is readily identifiable.
BENCHMARK C 174 (PID: GR0575) (NO ACCESS)
Unfortunately, I could not look for Benchmark C 174 as the coordinates placed it inside an active construction site. The datasheet lists the coordinates that put it on the plaza that is under renovation; however, a 1990 search for the mark reported that it could not be found. The original description of the mark indicates that it is 8 feet above the highway set on a rock ledge. That description contradicts the coordinates, so it's hard to know. The only surefire way to know will be to visit the site once it is open again and search the rock ledge for the benchmark.
TRIANGULATION STATION HOOVER DAM BLOCK T=V 4 (PID: GR0577)
Fortunately, this Triangulation Station disc was outside the construction fence! According to the datasheet, the disc is stamped with V 4 below the prestamped triangle and 1935 above it; however, due to wear, you can only make out the "4."
USC&GS Triangulation Station discs will be oriented so the top of the disc points to True North. The top of the disc would be determined by the triangle that points to the space between the "E" and "O" in Geodetic. I've tested this on countless discs, and it has proved accurate.
If you find an Azimuth Marker or a Reference Marker, the arrow on the mark will ALWAYS point to the station it references. Azimuth Marks are established at least one-half mile away from the primary station; Reference Marks will be within line-of-sight of the station mark and typically within 10-50 feet of the station.
BENCHMARK E 174 (PID: GR0576)
My final recovery at the dam was BOR Benchmark E 174, which was in excellent condition! With twenty survey marks recovered, I hit the cafe for lunch and signed up for a dam tour. 😊
After a full day at the dam, I packed back into the Jeep and headed home, wrapping up a fun and informative three-day mini-trip to Boulder City and the Hoover Dam—just the break I needed!
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