GMU 505 Mossyrock
A top-five District 10 blacktail unit on a jigsaw of small private ground around Riffe and Mayfield lakes.
WDFW Region 5 — Southwest Washington (District 10, Lewis County) · Last reviewed 2026-07-12
Area
298
sq mi
Public land
20%
approx.
Elevation
400–3,000
feet
Motorized
moderate
access
Overview
GMU 505 (Mossyrock) covers roughly 295 square miles east of Interstate 5 and north of the Cowlitz River, entirely within Lewis County. Unlike the big industrial-timber units to the west and east, its defining feature is a mosaic of small private agricultural lands and rural residences with timberland interspersed, wrapped around Riffe and Mayfield lakes on the Cowlitz River. It is one of District 10's most productive black-tailed deer units — WDFW ranks it among the district's top general-season buck GMUs — while elk are limited. The catch is access: dozens of small landowners make permission the whole game, though the Cowlitz Wildlife Area's Peterman Hill and Swofford Pond units give hunters reliable public deer ground.
Terrain
Mostly flatter to rolling topography of farm valleys, rural benches, and low timbered hills, with pockets of steeper, more rugged ground on the north and east edges. The Cowlitz River and its impoundments — Mayfield Lake and the larger Riffe Lake behind Mossyrock Dam — bisect the unit.
Cover: A working-landscape patchwork: pasture and hay fields, rural homesites, replanted clearcuts and second-growth Douglas-fir/hemlock timber, and brushy reprod along the timbered fringes. Wet maritime greenup keeps clearcut edges and field margins productive for deer.
Landmarks: Riffe Lake (Mossyrock Dam) · Mayfield Lake · Cowlitz River · Peterman Hill / Peterman Ridge (Cowlitz Wildlife Area) · Swofford Pond · Interstate 5 (western boundary) · US 12 / SR 12 corridor · Town of Mossyrock
Access
WDFW rates access here as challenging because the unit is chopped into numerous small private landholdings and rural residences — securing landowner permission is the central task. The bright spot is the Cowlitz Wildlife Area: the Peterman Hill (Peterman Ridge) unit provides productive public deer hunting and the Swofford Pond unit adds more open ground. Interspersed industrial timber follows the standard District 10 rules — Weyerhaeuser requires a paid recreation permit (roughly $250–$400 motorized, $75–$125 walk-in) except where enrolled in WDFW's Private Lands Access program, while other companies are generally open without a fee but may gate roads or close for fire danger in August–October. Primary vehicle access is off SR 12 and Peterman Hill Road.
Managers: Small private agricultural owners and rural residences, Private industrial timber (Weyerhaeuser and others), WDFW — Cowlitz Wildlife Area (Peterman Hill, Swofford Pond units), Washington DNR / state forestlands
Access points: SR 12 corridor through Mossyrock and Salkum · Peterman Hill Road into the Cowlitz Wildlife Area · Swofford Pond Road / Swofford unit · US 12 / Interstate 5 interchange at the west edge · County roads off SR 12 to private-land access points
Species notes
Blacktail Deer
common opportunityThe marquee species here. WDFW lists Mossyrock among District 10's highest general-season buck harvests (a five-year modern-firearm average near 340 bucks at roughly 29% success across ~890 hunters). It's a high-opportunity, hard-to-trophy blacktail hunt — plenty of deer, but mature bucks are the exception. The productive mix of farmland edges, clearcuts, and rural greenbelts holds deer, so the real work is turning that opportunity into legal access.
- →Hunt the Peterman Hill unit of the Cowlitz Wildlife Area for dependable public deer ground away from the permission grind
- →Glass field edges, clearcut greenup, and orchard/pasture margins at first and last light
- →Knock on doors well before the season — small private parcels are the difference-maker and a courteous ask often opens ground the road crowd skips
- →Still-hunt brushy timber stringers and creek bottoms between open fields on quiet, damp mornings
- →Late-season rut and post-rain movement pull bucks onto south-facing greenup along field lines
Roosevelt Elk
limited lowElk are limited in Mossyrock — this is not a herd hunt. The unit sits at the low, western fringe of the Mount St. Helens elk herd, and general-season bull harvest here is in the single digits with very low success. Treat elk as an incidental encounter, not the reason you draw a Mossyrock plan.
- →Focus elk effort in the timbered north/east edges and larger private blocks rather than the farm valleys
- →Cut roads to find fresh sign, then still-hunt the drainage holding animals
- →Note that Treponeme-associated hoof disease (TAHD) is common in District 10 elk — report and leave affected hooves in the field
Black Bear
common opportunityBlack bear are common in the unit's timbered and clearcut ground, drawn to spring/summer greenup and the fall berry crop. A solid opportunity species while deer hunting the wetter timber.
- →Glass clearcuts and reprod for bears feeding on grass, forbs, and ripening berries
- →Work sunny cut faces morning and evening, then move in for a stalk
- →Submit the required black bear premolar tooth to WDFW after a fall harvest
Cougar
common opportunityCougars follow the strong deer base through the unit's timber and greenbelts. Encounters are mostly incidental for deer hunters; dedicated cougar hunting is low-density and opportunistic under a seasonal harvest cap.
- →Hunt cougars where deer concentrate — the same field edges and timber pockets
- →Cut for tracks on muddy roads after weather
- →Verify the cougar area is still open before hunting late in the season
Pro tips
- →Solve permission first — this unit lives and dies on small private parcels, so line up landowner access weeks ahead and treat the Cowlitz Wildlife Area (Peterman Hill, Swofford Pond) as your reliable public fallback.
- →Play the opportunity, not the trophy: Mossyrock produces numbers of bucks, so hunt hard for a legal deer rather than holding out for a rare big one.
- →Glass the farm-and-timber transition at dawn and dusk — deer bed in cover and feed the open edges.
- →Buy any Weyerhaeuser permit and download its gate map before the season; know which interspersed timber is free-access and which is not.
- →Late-season rains and rut concentrate bucks on south-facing greenup along field margins — time trips to weather breaks.
Watch out for
- !Access is the biggest pitfall: numerous small private landholdings and rural residences mean trespass risk is high — confirm boundaries and permission on offline maps.
- !Weyerhaeuser ground requires a paid permit except where enrolled in WDFW Private Lands Access; a permit does not open every road.
- !Industrial timber may be closed for fire danger in late summer/early fall, and gates can appear without notice.
- !Elk are limited here — don't build an elk trip around Mossyrock; the Mount St. Helens herd is stressed and antlerless opportunity has been cut.
- !TAHD (elk hoof disease) is present in District 10 elk; follow WDFW reporting and hoof-disposal guidance.
- !Watch firearm-restriction rules near the rural residential and lakeside developments.
Logistics
Base out of Mossyrock or Morton for the interior, or the I-5 towns of Chehalis and Centralia for fuel, lodging, and supplies. Developed camping and boat access ring Riffe and Mayfield lakes (Tacoma Power / Cowlitz Wildlife Area facilities). Sort out private-land permission and any Weyerhaeuser permit before you arrive, and carry offline maps with land-ownership layers — the small-parcel checkerboard is easy to violate by accident. Cell coverage is decent near the SR 12 corridor and spotty in the back timber.
Nearest towns: Mossyrock, Morton, Salkum, Mayfield, Toledo, Chehalis, Centralia
Seasons & regulations
This page carries no season dates or regulations. Get current rules from the official Washington sources:
Research sources: WDFW — 2025 District 10 Hunting Prospects (Region 5) · WDFW — Game Management Units (GMUs) · WDFW — Elk Hoof Disease (TAHD) · GoHunt — Washington Unit 505 (Mossyrock) Profile
FAQ
- How big is GMU 505 in Washington?
- GMU 505 (Mossyrock) covers about 298 square miles, ranging from roughly 400 to 3,000 feet in elevation. Area is computed from the unit boundary polygon.
- How much public land is in GMU 505?
- Approximately 20% of the unit is public land. Primary managers: Small private agricultural owners and rural residences, Private industrial timber (Weyerhaeuser and others), WDFW — Cowlitz Wildlife Area (Peterman Hill, Swofford Pond units), Washington DNR / state forestlands. Always verify ownership boundaries on the ground.
- What species does the catalog list for GMU 505?
- Blacktail Deer, Roosevelt Elk, Black Bear, Cougar. For seasons, tags, and regulations, use the official Washington agency resources linked on this page — this page carries no regulatory information.
Put GMU 505 on your map.
Boundary lines, public/private ownership, topo, and offline maps for this unit — in Baseline Maps, free for 14 days.