There are a few parts to this question. One is about the reliability of Jacob Riis. In short, yes, I think the dives were substantially as bad as he makes out, but not necessarily because Riis is an infallible source. Before writing How the Other Half Lives in 1890, Riis spent a decade as a police reporter where he partook in his share of sensationalism and moralizing, features also present in Other Half. So it's certainly valid to read his work critically and corroborate his stories.

Having grown up a poor immigrant, Riis empathized with his subjects, but within limits: "It happens too often," Riis warns, that upon providing aid to a needy family one will discover "'sickness' to stand for laziness, and the destitution to be the family's stock in trade." While he criticized slumlords, his message to the propertied class was one advocating personal responsibility, not radical change. He was also a nativist. Being self-made, he disdained groups that seemed to be in perpetual poverty and, consequently, his book is full of racist stereotypes. In particular he scorned the newest immigrant groups to NYC like the Chinese and, in the case of the stale beer dives, Italians.

Dive Conditions

The stale beer dives didn't leave behind many official records. They were informal, unlicensed operations (hence the police raid Riis accompanied) and are therefore difficult to study in depth. Much of the later writing on the dives uses Riis as its primary source, but there are several other contemporary sources we can consider. At least as early as Charles Dickens' American Notes (1842), a book Riis apparently admired, and over the ensuing decades, there was a tradition of journalists, philanthropists and reformers who drummed up "sunshine and shadows" stories by taking (usually police chaperoned) tours of the city's slums.

In the 1890s several visitors wrote about the infamous "Mulberry Bend" block that Riis described. Through an alley and down some stairs behind 39 Mulberry St, political reformer Rev. Charles Parkhurst found a "dingy hole" of a dive bar where "Italians from Calabria" sat on the dirt floor, playing cards, "ready to cut your throat for a shilling." (Here's a picture of Parkhurst "in disguise" and ready for his adventure.) Social activist Helen Campbell visited a nearby dive kept by "Rosa, an Italian woman" and described an empty cellar "save a bed, a stove, and benches around the walls... A crowd of men and women in all stages of drunkenness sat about on the benches, some listening to 'Accordeon Mary' playing an asthmatic accordeon, some singing to it."

Complimenting these accounts was a trend in police and crime reporting that emerged in the mid 19th century. In newspapers we can find plenty of references to the stale beer dives, in a reporter's own excursion into the slums, and in several mentions of police raids on Mulberry Bend.

Lest we doubt the dire conditions of New York's slums, we can look at a source that wasn't written for the commercial media: the Council of Hygiene and Public Health's 1865 report which had doctors survey every building in Manhattan. A section about Mulberry Bend describes an "old Baptist church transformed into a Tenant-house" housing fifty-nine families consisting of 313 people. Fifteen people had died in the house in the preceding year, half of them children. One in four residents were "constantly sick." We also learn that it was on this block that the 1849 cholera epidemic started.

This was typical of the tenements. In the densest districts a two-room apartment would regularly hold two or three immigrant families, plus lodgers.

Stale beer dives were found in "rear houses," units built behind the main building where there would normally be a yard or open space. Rear houses would butt up against the rear houses of adjoining lots, leaving little space for airflow or sunlight. These, along with cellar apartments, represented the worst-of-the-worst housing units in the city, a step down from shared rooms in the main tenement building.

The Council of Hygiene provided an illustration of what one of these overcrowded "fever nests" might look like, and laid out with excruciating detail exactly why these rear houses were epicenters of filth and disease. I personally find the matter-of-fact language of the report more disturbing than Parkhurst or Riis:

The privy, from necessity, is located in close proximity to the rear house, either immediately in front of a window, or just at the entrance to the cellar. [Overflowing] frequently happens after a hard rain... spreading the contents not only over the yard, but in some instances into the cellar, the bottom of which becomes covered with this semi-liquid filth. In other instances the seat and floor of the privy become soiled and filthy to such an extent that they are wholly unfit for use, and the poor tenants are compelled to resort to their chamber utensils, the contents of which are emptied into the garbage-box, into the already overfilled privy, or into the narrow space between the rear walls of the two houses. This space is so narrow in a majority of instances as to render it impossible for a man to pass between the walls, often becomes the receptacle for all rubbish, garbage, and filth of every description, creating an odor so offensive that it is necessary to keep windows closed, depriving the tenants of that source of ventilation.

There was some progress regulating housing between 1865 and when Riis toured the slums, but the new laws mostly proved toothless in practice. Riis himself discusses this in his book, and argues that such reports, once the shock had worn off, might have a numbing effect. "It no longer excites even passing attention, when the sanitary police report counting 101 adults and 91 children in a Crosby Street house."

So when Riis describes the floors of "hard-trodden mud" and walls "covered with a brown crust," he's not embellishing. And of course it helps that he provided his photos as evidence.