if you do end up digging, you can mininmize the damage.
One option would be to find a plumber that can use a piercing tool

Here is a link that might be useful: trenchless example

I am trying to visualize your layout and if I am correct you should have a third valve on the line that you may not have located. If I am correct your problem may be much easier than you think.

Typically the municipal main is located in an easement parallel to the roadway about 5' to 8' wide parallel to the roadway. (In areas where there are sidewalks the utility easement is in the treebelt between the roadway curb and the sidewalk.) There should be a "Curb box valve" located in that easement that defines the point of demarcation between the municipal main and the house "Supply Line". The curb box valve is a "Keyless hydrant" valve located totally underground. On the surface you should find a small round cast iron cover flush with the finished grade about 4" to 6" in diameter that is held in place by a hex head nut and the word "Water" is visible as raised embossed lettering on the cap. This is the valve that the water service provider would turn off to discontinue water service and while most plumbers have a curb box wrench, in some jurisdictions only bona fide employees of the water service company are legally permitted to have a curb box service wrench.

When the cast iron cap is removed it reveals a hollow pipe extending down to the valve which is on the line below grade. The service wrench is simply a steel rod 3" to 10" long with a Tee hnndle on one end and on the opposite end it has a U shaped piece of steel that fits over an extended flat steel bar on the valve. The rod is put down the pipe until it contacts the valve then it is turned a 1/4" turn to shut the valve off.

The buried line from the curb box valve to the structure is then classified as the "Supply Line". Typically for houses with a basement the water meter is located in the basement in the immediate vicinity of where the supply line enters the structure. For houses on slab the meter is typically installed in a buried Meter vault at any convenient point between the utility easement and the structure.

Code Requires that all structures must have a "Main Shutoff Valve" in close proximity to where the Supply line enters the structure. (Most codes define close proximity as a distance not more than 6'). For houses on slab the Main shutoff is typically installed in a Valve box immediately outside the foundation where the water line enters the building. (For houses with a basement the main shutoff is immediately inside the basement wall.) The "Main Shutoff" is the official point of demarcation between the house "Supply Line" and the house "Distribution lines". These demarcation points are important because code defines the size and type of pipe and materials we may use to install these lines and it may be radically different. By example PVC is permitted for Supply Piping but it is prohibited for Distribution piping.

The gate valve which you describe as being on a copper pipe just before it enters the house is your "Main Water Shutoff Valve". If this valve is in a valve box or vault that is immediately accessible it technically is not in a concealed location so you may be permitted to use copper compression fittings here(chech with your local code official). If so, the easy solution would be to cut the copper pipe and use compression fittings to install a new ball valve. If you can cut the old valve out while doing this thats great, but if not simply remove the handle from the old valve and leave the valve in place.

The second option would be to contact your water service provider and have them turn the service off temporarily to permit you to service the lines.

In a worst case scenario where you may need to replace the supply line it is not as difficult as you might imagine. Typically we would use a Ditch Witch type trenching machine which has a cutter bar that looks like a super chainsaw on steroids. The trencher cuts a 4" to 6" wide trench to the required depth of up to 6 or 8ft and some have an attachment on the bar that will actually lay the new pipe as it cuts the trench. Some of the machines also have a small backhoe bucket on the opposite end plus a small bulldozer blade which is used for backfilling. Your total run is 60 feet so the total trenching time would be in the order of about 10 to 15 minutes plus the time to lay the pipe and backfill with a minimal disturbance to your landscaping. Keep in mind that they would need to hand dig on each end to make the final connections plus the time to deliver and setup the machine so a total service time of 4 to 5 hours would be about normal.

The California Code is primarily based upon the Uniform Plumbing Code so your options of pipe materials should be continuos roll copper pipe, types K & L which is approved for direct burial, continuous roll Polyethelene pipe, PEX tubing, PEX-AL-PEX pipe, or PVC pipe.

Thx much for the response lazypup. My "street" configuration sounds a little different than what you describe. Picture of my curb box innards can be seen at http://www.geocities.com/runnernest/WSL_entry.jpg. The gate valve (circled) is the one stuck open.

The included link is a picture of the main shutoff valve, the valve that is broken open.

I won't know how much galvanized pipe there is in the service line until I start working my way back from the curb box to the house...at some point it will transition to copper, my guess is within a few feet of the house. I have a compact backhoe tractor so digging is no problem, other than the damage to the front yard.

I could do the ball valve with compression fittings at the service entry, but I should probably just go ahead and redo the entire service line with buried copper and full port ball valves.

Will this require a permit?

Here is a link that might be useful: {{gwi:2043516}}

there is likely a shutoff below grade where your line ties into the municipal line as lazypup said.
Call your local water company/agency and ask them to locate your curb stop for you. Around here they know about where it should be, and use a big magnet on a lanyard because the covers are metal.