Harvey Park, pre-settlers

As you can see from archival photos, Harvey Park of the 50s looked very different than the Harvey Park of today. The biggest difference, to my eyes, is that there were no trees.  Before that though, there were no houses, and Harvey Park was a ranch, roamed by herds of cattle. Before even that, Harvey Park was home to the native Colorado shortgrass prairie.

Though there are many plant species that make up the shortgrass prairie, the landscape would have been largely dominated by just these two warm-season, C4 grasses:
Blue gramma - Bouteloua gracilis

and
Buffalo grass - Bouteloua dactyloides

Though ~75% of the prairie biomass would have been these two grasses, there would have been plenty of other plants as well. There would have been a number of legumes, which, due to their symbiotic root bacteria known as rhizobia, help fertilize the soil by fixing nitrogen from the atmosphere. They would have included:
Purple prairie clover - Dalea purpurea

Blue wild indigo - Baptisia australis

and
Lead plant - Amorpha canescens

along with quite a few others. Finally, there would have been the forbs, which is a catchall group for any herbaceous plant that's not a grass or a legume. These would include members of the sunflower family:
Golden Aster - Chrysopsis villosa

Prairie Coneflower- Ratibida columnifera

Blazing star - Liatris punctata

The flax family:
Blue flax - Linum lewisii

The (now defunct) figwort family:
Indian paintbrush - Castilleja integra

Beardlip penstemon - Penstemon barbatus

The mallow family:
Cowboy's delight - Sphaelralcea coccinea

Winecups - Callirhoe involucrata

The verbena family:
Prairie verbena - Clandularia bipinnatifida

The evening primrose family:
Scarlet gaura - Gaura coccinea

as well as many, many, many others, in relatively small numbers.

As you are likely well aware, Harvey Park sits on a hill above Bear Creek. While the area immediately around the creek would have been lush, with willows, cottonwoods, rushes, horsetails, etc., the effects of the moisture wouldn't have been very far-reaching. In fact, Harvey Park South would have been a particularly severe landscape. The steep south-facing hillside means more sun exposure and less rainwater recharging than Harvey Park proper (north of Yale). Plus, being in a valley, Harvey Park South gets cool air masses descending out of the mountains at night, meaning later spring frosts and earlier fall frosts. This means it would have had more hardy perennials, like soapweed yucca, prickly pear cactus, and rabbitbrush.Harvey Park, being on a hilltop with gently north and east facing slopes, would have had a higher proportion of wildflowers and cool season grasses. Harvey Park pond, as well as the reservoirs, would not have existed.

For more information on the ecology of shortgrass prairies, you can check out CSU's long-term shortgrass prairie study site at Pawnee National Grasslands.