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Lake Sonoma 100K & 50K Course Guide

Lake Sonoma is a Wine Country ultra and it is rolling singletrack the whole way, run around the Warm Springs arm of the lake near Geyserville. You open with a big climb up Pritchett Peak, and then it is just constant ups and downs that never let you settle in, and a mid-April day that can turn warm on you. I will walk you through the course, and then give you the pacing and fueling that actually works out here, plus a few free tools so you can run your own numbers.

⏵ Quick facts

Lake Sonoma at a glance

Date
Mid-April, annual (next edition expected mid-April 2027; most recent confirmed edition Sat, April 11, 2026)
Location
Lake Sonoma, near Geyserville / Healdsburg, Sonoma County, CA
Distances
100K, 50K, and the Trail Sisters Women’s Half Marathon (13.1 mi)
100K elevation gain
Roughly 15,000 ft of climb, with about as much descent
Terrain
About 90% unpaved, mostly rolling singletrack around the lake
100K aid stations
13 total (8 full, 5 water-only)
100K cutoff
19 hours (finish under 19h for a Western States qualifier)
50K cutoff
11 hours
Qualifier
The 100K is a Western States Endurance Run qualifier

Note: Lake Sonoma is run annually in mid-April. The most recently confirmed edition was Saturday, April 11, 2026, and the next edition is expected around mid-April 2027. Dates, start times, the exact route, aid stations, and cutoffs can change year to year, so always confirm the current specifics on the official Run Lake Sonoma site before you plan your race.

The course

Lake Sonoma is almost all trail, roughly 90% unpaved, and it runs through madrone groves and lakefront paths and smooth California singletrack around the Warm Springs arm of the lake. The trail itself is not technical in the rocky, rooty sense. What gets you is that it never stops rolling. The 100K stacks roughly 15,000 feet of gain and about as much descent, and it does it with a ton of short, punchy climbs instead of a few big ones. The 50K is the same kind of trail on a shorter loop.

Pritchett Peak sets the tone

The 100K starts in the dark with a hard climb up Pritchett Peak, and you get a sunrise summit before the day really gets going. It is the single biggest sustained climb on the whole course, and it comes while your legs are still fresh, and that is the trap. It feels easy to push here. Pushing here is how people pay for it later, on a course that never gives your legs a real break.

So climb the opener by effort. Hike the steep stuff with purpose and save your running legs for the rolling singletrack that comes after. There is a second decent climb up toward the South Lake trailhead before the long middle of the race settles into that rhythm of constant ups and downs.

Relentless rolling, not one big grind

After the early climbs, the thing about Lake Sonoma is that it just never flattens out. The trail rolls the whole way around the lake, up and down through the trees, and there are very few stretches where you can switch off and cruise. Each climb on its own is short. But they keep coming, and it is that piled-up gain that wears you down over the full distance.

The footing is smooth and runnable, so you will want to race every roller hard. Do not. The people who finish strong are the ones who run the climbs at a controlled, repeatable effort and let the descents come to them, instead of attacking every little bump and showing up to the back half with nothing left.

Where the race is won or lost

Lake Sonoma gets decided on the descents and in the warm middle of the day. The total descent roughly matches all that gain, so the constant downhill running quietly shreds your quads if you are not ready for it, and once your quads are gone on this course, every remaining roller turns into a grind. Honestly, quad-specific downhill training is about the most race-specific work you can do for this one.

The other thing is the weather. Mid-April in Sonoma County can be cool or it can be genuinely warm, and the exposed lower sections later in the day are where the heat catches under-fueled runners. There are creek crossings to deal with too. Staying on top of fluid, sodium, and calories through the warm hours is what keeps the back half from falling apart.

Aid stations and cutoffs

The 100K has 13 aid stations in total, 8 full and 5 water-only, plus drop bags at the major checkpoints and crew access at a handful of them. The overall 100K time limit is 19 hours, and finishing under that 19 hours is what gets you the Western States qualifier. The 50K runs its own set of aid stations with an 11 hour cutoff.

There are cutoffs at checkpoints along the way on the 100K, so you cannot just hike the front half casually. Pull the official Run Lake Sonoma checkpoint cutoff chart for the current year and build your plan backward from those times, and leave yourself a buffer, because all that rolling and any heat will slow you down more than you think late in the day.

Pacing strategy for Lake Sonoma

A course that rolls this much rewards a steady, repeatable effort, and it punishes anyone who races every climb. Pace Lake Sonoma by effort and by grade, not by the flat-ground numbers from your home runs.

Run the rollers by effort, not by clock

On a course with roughly 15,000 feet of gain spread across endless short climbs, your moving pace is going to swing all over the place, and that is fine. Power-hike or run the climbs at a controlled, repeatable effort and let the descents flow, instead of chasing some fixed minutes-per-mile number the terrain is never going to let you hold.

Use our free grade-adjusted pace calculator to turn your flat fitness into honest effort targets for the Lake Sonoma climbs, so you can tell whether you are running the rollers in a way you can keep up, or burning matches you will want back in the final hours.

Protect your quads for the descents

The descending roughly matches all that climbing, so downhill running is the thing that quietly decides your day. Run the early descents light and controlled instead of letting gravity pound your legs, and your back half will be so much better for it. The people who hold their pace late are usually the ones who still have working quads at 40 miles into the 100K.

To set a finish goal that actually accounts for all that rolling vert, use our vert-aware race time calculator. It works the climbing into your projected finish so you are not stuck on some flat-course estimate that the Lake Sonoma rollers will quietly tear apart.

Plan for the warm middle and the cutoffs

Mid-April here can turn warm, and the hottest hours usually land in the exposed middle of the day. Bank a little time early, but do not cook yourself on the opening climb doing it. Then plan to slow down in the heat, and protect your margin against the cutoffs so the warm hours do not put you behind the clock.

If you want to know how your fitness from a recent race carries over to a 100K or 50K effort on this kind of rolling singletrack, our race equivalent calculator helps you reality-check your goal before you commit to a finish time.

Fueling strategy for Lake Sonoma

A long, constantly rolling, often-warm day makes your fueling and hydration matter just as much as your fitness. The mid-April warmth is the thing that wrecks otherwise well-trained runners, so plan for it.

Carbs: a steady drip, on a trained gut

For a day this long and this constantly rolling, aim for roughly 60 to 90 grams of carbohydrate per hour, and lean toward the high end once your gut is trained to handle it. Use a glucose-plus-fructose blend so you can absorb more than a single sugar lets you, and practice your exact hourly carb number on your long runs so race day feels normal and not like an experiment.

The climbs never really stop, so your engine is always working, which means a steady drip of calories beats a few big feeds. Keep eating through the warm middle hours, when your appetite drops off but the effort does not. That is exactly where most people quietly fall behind on fuel.

Sodium and fluid: built for a warm day

Plan your hydration for the warmer day. Aim your sodium toward 500 to 700 mg per liter of fluid and carry enough to get you through the gaps between aid stations, especially on the lower, more exposed sections later on. Cramping, a sloshy stomach, that wrung-out feeling late in the race, those are usually fluid and sodium problems, not fitness problems.

Dial in your own plan with our free ultra fueling calculator. Put in your weight, your goal time, and the heat you expect, and it gives you a carb, sodium, fluid, and caffeine plan per hour built for the Lake Sonoma distance and conditions. Then go test it in training.

⏵ Free calculators for this race

Run your Lake Sonoma numbers with our free, no-signup tools, then let Summit Line tie them all back to your real fitness and this exact course.

See all free running tools →

Train for the distance + terrain

Lake Sonoma asks for rolling-vert legs, downhill-proof quads, and a fueling plan you can hold for hours. These free guides go deeper on each piece, the 100K or the 50K, whichever one you are chasing.

⏵ Train for Lake Sonoma

Get a race-day plan dialed to YOUR fitness, this exact course, and your projected splits. Summit Line reads your actual training, builds a fueling and pacing plan around the Lake Sonoma rollers and the mid-April warmth, and tracks how your gut and legs handle the load, so race day is rehearsed and not guessed.

Get started free →See it on a real ultra

Lake Sonoma 100K & 50K FAQ

How hard is the Lake Sonoma 100K & 50K?

The 100K is a hard mountain ultra, no way around it. It is hilly and remote, with roughly 15,000 feet of climbing and about as much descending across mostly rolling singletrack around the Warm Springs arm of Lake Sonoma. It opens with a big climb up Pritchett Peak in the dark, and then it never really flattens out, so you never get a long stretch of easy running to recover on. Add a 19 hour cutoff and a mid-April day that can turn warm, and the 100K earns its Western States qualifier. The 50K is shorter but it is built from the same up-and-down trail, with an 11 hour cutoff, so it is a serious day too. The Trail Sisters Half Marathon is the most doable of the three, but it still runs the same hilly terrain.

How much climbing is in the Lake Sonoma 100K?

The 100K carries roughly 15,000 feet of total elevation gain and about the same amount of descent. The single biggest sustained climb comes early, up Pritchett Peak for a sunrise summit, with another good climb up to the South Lake trailhead. After that, the course is not really about one giant climb. It is constant rolling gain and loss on singletrack, short, punchy ups and downs that add up to that big total. The 50K is the same kind of rolling, so both distances want legs trained on repeated short climbs and quad-pounding descents, not one long grind.

How should I fuel for the Lake Sonoma 100K & 50K?

Fuel for a long, rolling, often-warm day. Most ultrarunners aim for 60 to 90 grams of carbohydrate per hour, leaning toward the high end once the gut is trained, and a sodium concentration around 500 to 700 mg per liter of fluid. Mid-April in Sonoma County can be cool or genuinely warm depending on the year, so plan your fluid and sodium for the warmer case and dial it back if it stays mild. The constant climbing keeps your effort high, so a steady drip of calories matters more than a few big feeds. Our free ultra fueling calculator turns your weight, goal time, and the heat you expect into a carb, sodium, and fluid plan per hour.

What are the Lake Sonoma cutoffs?

The 100K has a 19 hour overall time limit, and finishing under that 19 hours is what gets you the Western States qualifier. The 50K has an 11 hour cutoff. The Trail Sisters Half Marathon runs with a generous limit of several hours plus a cutoff at the Bummer aid station. There are cutoffs at aid stations along the 100K too, so you cannot just hike the front half casually. Always confirm the start times and the current checkpoint cutoff chart on the official Run Lake Sonoma site before you build your plan.

Is the Lake Sonoma 100K a Western States qualifier?

Yes. The 100K counts as a Western States Endurance Run qualifying race when you finish inside the 19 hour cutoff. That qualifier status, plus the Wine Country setting and all that singletrack, is a big part of why the race draws a strong field every April. The 50K and the Trail Sisters Half Marathon are not the qualifying distance, so if a Western States ticket is what you are after, the 100K is the one to enter.

What is the terrain like at Lake Sonoma?

Almost all trail. The course is roughly 90% unpaved, running through madrone groves, lakefront paths, and smooth California singletrack around the lake, with only short paved or dirt-road connectors. The trail itself is runnable and not especially rocky or rooty. What gets you is that it never stops rolling, so the difficulty is the sheer number of short climbs and descents, not the footing. Expect creek crossings and some exposed, warmer sections later in the day. It rewards strong, controlled downhill running and good power-hiking on the climbs.

This guide is for planning and training purposes and reflects publicly available information about the Lake Sonoma 100K & 50K. Race details, including the date, course, aid stations, and cutoffs, can change year to year, and some specifics for an upcoming edition may not be final. Always confirm the current specifics on the official Run Lake Sonoma race website before you train or travel.