“The Fourth of July is yours, not mine,” sings Ned Peters, the freed black slave. We're in 1851, at the point of the California Gold Rush where the easily dug gold is beginning to run out and things are turning ugly. White Americans are turning on everyone else – violently forcing out blacks, Chinese and Latinos (the extermination of the native peoples would follow). In their latest opera, Girls of the Golden West, John Adams and Peter Sellars demand that their audience take a cold, hard look at the dark side of the founding of California as a US state.

If Adams and Sellars were visual artists, they would be Breugel. The opera is a sprawling canvas inhabited by characters of many types from many backgrounds, engaged in the rowdiness, the tawdriness, the youthful exuberance, the lewdness, the hardships and – in the end – the plain violence of those lawless days. To portray those characters, they employ an exceptional cast of young singers, all American bar Korean Hye Jung Lee. Adams' vocal writing is tough on the singers, with myriad key changes, rhythmic shifts and difficult intervals, but these singers power through it with aplomb and show complete commitment to their varied roles.

The choice of different voice types is carefully crafted. Lee, as the prostitute Ah Sing, produces high wire coloratura as dazzling as Fourth of July fireworks. Julia Bullock sings Dame Shirley, the author whose descriptions of the period form the basis of the opera. It's a detached narrator role – overwhelmingly, she is commenting on the action, not engaged in it – but Bullock's performance is anything but detached, with lyrical beauty and purity of voice underpinning deep empathy for the people she observes. J'Nai Bridges sings the ill-fated Josefa with pride, elegance and a gloriously rich mezzo (as well as looking stunning), her dignity reminds us that the Mexicans were the established upper class of California before the “Argonauts” arrived and of the unceremonious way in which the Anglos displaced the “greasers”, as they called them, from land and power. I found the writing for male voice less distinctive, but all of Davóne Tines, Paul Appleby, Elliot Madore and Ryan McKinny showed attractive voices and the same intensity of character impersonation: Tines with the swagger of the freed slave, Appleby as the archetypal Argonaut chancer Joe Cannon, Madore as Josefa's lover Ramón, McKinny as Clarence, another narrator role but from the Anglo side.

The score is identifiably Adams while not especially similar to any of his others. The similarity is in the insistent, repetitive rhythmic drive that keeps the music propelled forwards, with the varied instrumentation of the figures above that rhythmic underpinning. The difference is in the depth of intensity and dark beauty of the lyrical passages: Ned's soliloquy and Shirley's commentary on it, which closes Act 1, is a particularly fine example, as is the love duet between Josefa and Ramón in Act 2. Under the baton of Grant Gershon, the SF Opera Orchestra played the score vividly and accurately, my one concern being that the War Memorial Opera House isn't the kindest of spaces to young singers whose voices are not huge, and they struggled to be heard above the orchestra in Act 1, before things warmed up.

Sellars and set designer David Gropman opt for a blank, somewhat Brechtian, stage into which props of various sizes are brought as required by the many scene changes – the sizes ranging from a simple chair to a wagon to a giant sawn-off tree which is initially used as the platform for a performance of Macbeth – one of the strangenesses of the Gold Rush being that the frontier roughness didn't prevent theatre, opera and other arts from flourishing (the money to pay for this, of course, was not an issue).

For all its narrative power, Girls of the Golden West suffers from two problems which I believe to be significant. The first is that the vast majority of the text is narration – and unlike, say, Il trovatore, which has been accused of the same thing, the majority of that is spoken by the narrator directly to the audience, not to the other characters. Act 1, where we are being introduced to the scene and the characters involved, began to feel like a very long history lecture. The second problem – to my taste, at least – is that the opera is very much split into two contrasting halves: the brash optimism and general weirdness of the Gold Rush environment in the first half, and the ensuing dark side of racism and violence in the second. There was little intermingling of light and shade here: we got light, then dark. At three hours and twenty minutes, that contributed to making the opera feel half an hour or so too long.

San Francisco Opera have given us an important opera, which is musically demanding and which challenges Americans to understand their history and, by extension, the troubled issues of wealth and race in the US today. It deserves to be seen.