Soft Rush (Juncus effusus) in western North America. This is a big, tough plant with one bract extending above the inflorescence (flower cluster) like a continuation of the stem, so the inflorescence looks like it’s coming out of the side of the stem. Because many experienced field botanists and ecologists recognize this big wetland plant but few are familiar with recent introductions or recent taxonomic changes, the name is happily applied to a whole group of species and all the J. effusus subspecies, most of which can be distinguished if the right parts are photographed.
What do you need? Whole plant, stem width (wide or narrow), and leaf sheath tops. Yes, leaf sheath tops. They’re near the base of the plant. They’re hard to get out because the clump is so dense, but dig down there and maybe use a knife. Also get a decent close-up of a flower if you can, so length of tepal and capsule can be compared.
Good references: Flora of the Pacific Northwest, 2nd Edition. Jepson Manual of the Higher Plants of California, especially the on-line version. Articles on the group by Peter Zika. NOT Flora of North America.
Examples with leaf sheath tops. Wide stems: https://www.inaturalist.org/observations/23713392 and https://www.inaturalist.org/observations/28540561 . Narrow stems: https://www.inaturalist.org/observations/35970101 and https://www.inaturalist.org/observations/35970101 . That last has a comparison of J. effusus with wide stems and asymmetrical leaf sheath tops, and J. laccatus with narrow stems and symmetrical leaf sheath tops.
By the way, the subspecies of Juncus effusus are important to determine because we have both native and introduced forms. The introduced invasive forms are sometimes planted in wetland restorations.