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Isle Royale National Park

Michigan · Established April 3, 1940

Ancient lava ridges, wolves, and moose on a wilderness island in Lake Superior

Area: 571,790 acres
Established: April 3, 1940
Recreation visits: 29,091 / yr
State: Michigan
Highest point: Mount Desor, 1,394 ft (425 m)
Lowest point: Lake Superior shoreline, about 600 ft (183 m) above sea level

Isle Royale Weather Forecast

Loading the National Weather Service forecast for Isle Royale…

Seven-day outlook at the park's reference point (48.011, -88.828). Elevation changes everything in the mountains — expect colder, windier and wetter conditions on high ground.

Map of Isle Royale National Park

The Park

Isle Royale rises from the cold blue expanse of Lake Superior like the spine of a sleeping animal: a long, narrow island of parallel ridges and fjord-like harbors, surrounded by some 400 smaller islands. There are no roads. Visitors arrive by ferry or floatplane and travel on foot or by boat, and the park closes entirely each winter when the lake turns violent and ice-bound.

The island is best known for a natural drama played out in isolation. Moose reached the island in the early 1900s and wolves around 1949, and the relationship between predator and prey here has been studied continuously since 1958, one of the longest-running studies of its kind in the world. Around them, a cool, lake-moderated climate supports boreal forest of spruce, fir, and birch, orchids and carnivorous plants in bogs, and loons calling across quiet inland lakes.

Geography

Isle Royale is about 45 miles long and 9 miles wide, lying closer to Minnesota and Ontario than to Michigan, the state to which it belongs. The park takes in the main island, hundreds of smaller islands, and the surrounding waters of Lake Superior out to about four and a half miles, so most of its roughly 894 square miles is water.

Its terrain is a series of long ridges and valleys running southwest to northeast, the eroded edges of tilted lava flows and conglomerate layers. The Greenstone Ridge forms the island's backbone and carries the main trail over Mount Desor, the high point at 1,394 feet. The Minong Ridge parallels it to the north, while deep, narrow harbors such as Washington Harbor, Chippewa Harbor, and the waters around Rock Harbor penetrate the coastline.

Dozens of inland lakes fill the valleys between ridges. Siskiwit Lake, the largest, holds islands of its own, one of the curiosities of a place where lakes within an island within a lake contain still smaller islands.

Landmarks

Mount DesorPeak · 1,394 ftThe highest point on Isle Royale, a wooded summit crossed by the Greenstone Ridge Trail.
Greenstone RidgeAreaThe island's central spine of tilted lava flows, followed by a trail for most of the island's length.
Minong RidgeAreaA rugged northern ridge with one of the park's most demanding trails.
Feldtmann RidgeAreaA ridge in the island's southwest with long views over Lake Superior.
WindigoAreaThe western gateway and ranger station at the head of Washington Harbor.
Washington HarborCoastA long, sheltered harbor at the island's southwestern end.
Rock HarborCoastThe island's main eastern hub, with a lodge, marina, and access to Scoville Point and the Greenstone Ridge.
Scoville PointCoastA rocky point at the end of a popular day hike from Rock Harbor.
Chippewa HarborCoastA deep, narrow inlet on the south shore reached by trail or boat.
Lake RichieLakeAn inland lake known for loons, moose, and paddle routes.
Feldtmann LakeLakeA quiet lake in the southwest, a good place to watch for moose.
Siskiwit FallsWaterfallA small falls on the Siskiwit River near its outlet toward Siskiwit Bay.
Mount SiskiwitPeakA high point along the Greenstone Ridge in the island's center.
Rock Harbor LighthouseCoastAn 1855 lighthouse guarding the entrance to Rock Harbor.
Passage Island LightIslandA lighthouse on a small island off the northeastern tip of Isle Royale.
Rock of Ages LightIslandA lighthouse on a reef off the western end, marking hazardous shipping waters.

Geology

About 1.1 billion years ago, the North American continent began to tear apart along a great arc now called the Midcontinent Rift. As the crust stretched and thinned, enormous volumes of basalt lava welled up and flooded the widening rift in flow after flow. On Isle Royale these lavas, known as the Portage Lake Volcanics, include roughly 100 individual flows stacked at least 10,000 feet thick.

Rivers flowing into the rift then washed sand and gravel over the volcanic pile, forming the Copper Harbor Conglomerate. The rift eventually failed before an ocean could open, and the heavy basalts sagged into a basin beneath what is now Lake Superior. As they did, the layers on Isle Royale were tilted gently toward the southeast, while matching rocks on Michigan's Keweenaw Peninsula tilted in the opposite direction.

Erosion over hundreds of millions of years picked out the difference between hard and soft layers. Resistant lava flows form the island's ridges with steep northwest faces and gentle southeast slopes, and softer layers wore into the valleys, harbors, and lakes. Pleistocene ice sheets scoured the ridges, deepened the valleys, and left the island's thin soils; as the ice retreated, changing lake levels carved ancient beach terraces that now stand high above today's shore.

Minerals deposited in gas bubbles and fractures within the lava give Isle Royale its gemstones, including chlorastrolite, or Isle Royale greenstone, which is Michigan's state gem. Native copper in the same rocks drew Indigenous miners thousands of years ago, and later 19th-century mining ventures. Collecting any rock or mineral in the park is prohibited.

Deep time

About 1.1 billion years agoThe Midcontinent Rift opens and floods the region with basalt lava flows.
Shortly after riftingRivers deposit sand and gravel that become the Copper Harbor Conglomerate.
Late PrecambrianThe rift fails and the rocks sag and tilt, forming the Lake Superior basin.
Hundreds of millions of yearsErosion carves ridges from resistant lava flows and valleys from softer layers.
Pleistocene Ice AgeContinental glaciers scour the island and deepen the basin of Lake Superior.
About 10,000 years ago onwardThe island emerges from glacial lakes; old shorelines are left stranded on its slopes.
Early 1900sMoose arrive on the island, followed by wolves around 1949.

The rocks

Portage Lake VolcanicsMesoproterozoic, about 1.1 billion years
Flood basalts that form most of the island and its ridges.
Copper Harbor ConglomerateMesoproterozoic
Rift-filling gravels and sands exposed along the island's southwest shores.
Greenstone FlowMesoproterozoic
A remarkably thick basalt flow that underlies much of the Greenstone Ridge.
Chlorastrolite amygdulesMesoproterozoic
Green, turtle-backed pumpellyite crystals filling gas pockets in the lava, Michigan's state gem.
Glacial and beach depositsPleistocene to Holocene
Till, outwash, and raised beach ridges left by the ice sheets and higher ancient lake levels.

Trees, Plants & Ecosystems

808vascular plant species recorded

Boreal forestShoreline and cool uplands, about 600–1,000 ft
Balsam fir, white spruce, paper birch, and aspen dominate the lake-chilled eastern end and coasts.
Northern hardwood forestInterior ridges, about 800–1,394 ft
Sugar maple and yellow birch grow on warmer interior ridges in the island's southwest.
Rocky shorelines and ridge topsExposed bedrock
Wind-swept rock supports lichens, juniper, bearberry, and arctic-alpine plants that are relics of colder times.
Bogs, swamps, and beaver pondsValley bottoms
Black spruce, tamarack, cedar, pitcher plants, and sundews fill wet valleys shaped by beavers.
Inland lakes and Lake SuperiorAquatic
Cold, clear waters hold lake trout, coaster brook trout, and ciscos, with loons nesting on quiet shores.

Trees

Balsam firAbies balsameaA fragrant boreal conifer and a key winter food for moose.
White sprucePicea glaucaA tall spruce of cool, lake-influenced shores and forests.
Black sprucePicea marianaThe slender spruce of the island's bogs.
Paper birchBetula papyriferaWhite-barked birch that colors ridgetops gold in autumn.
Quaking aspenPopulus tremuloidesColonizes burned and open ground; a favorite food of beavers.
Sugar mapleAcer saccharumDominates the northern hardwood forests of the interior ridges.
Yellow birchBetula alleghaniensisA bronze-barked birch of the maple forests.
Northern white cedarThuja occidentalisGrows in shoreline and swamp forests, where moose browse its lower branches.
American mountain ashSorbus decoraBright red berry clusters feed birds in late summer.
White pinePinus strobusA tall pine that persists in scattered stands on the island.
Red pinePinus resinosaA fire-adapted pine of dry, rocky sites.
TamarackLarix laricinaA deciduous conifer of bogs that turns golden before dropping its needles.

Wildflowers, shrubs & other plants

Devil's clubOplopanax horridusA spiny shrub whose main range is the Pacific Northwest, surviving here as a disjunct population.
ThimbleberryRubus parviflorusBroad-leaved shrubs with soft red berries line trails in late summer.
Mountain mapleAcer spicatumAn understory shrub-tree heavily browsed by moose.
Canada yewTaxus canadensisOnce widespread in the understory, it survives mainly where moose cannot reach it.
BunchberryCornus canadensisA tiny dogwood that carpets the forest floor with white flowers and red berries.
Bluebead lilyClintonia borealisYellow spring flowers followed by porcelain-blue berries.
TwinflowerLinnaea borealisPaired pink bells nod over mossy boreal forest floors.
Calypso orchidCalypso bulbosaA small, exquisite pink orchid of cool conifer forest.
Pink lady's slipperCypripedium acauleA showy orchid of acidic woods and bog edges.
ButterwortPinguicula vulgarisA carnivorous plant of wet shoreline rocks, a relic of colder climates.
Purple pitcher plantSarracenia purpureaTraps insects in water-filled leaves in the island's bogs.
Round-leaved sundewDrosera rotundifoliaSticky, glistening leaves snare tiny insects on sphagnum moss.
Lowbush blueberryVaccinium angustifoliumRipens on rocky ridgetops in midsummer.
BearberryArctostaphylos uva-ursiA trailing evergreen of sunny rock outcrops.

Wildlife

18mammals
243birds
3reptiles
8amphibians
53fish

Mammals

MooseAlces alcesArrived in the early 1900s and now the island's dominant herbivore.
Gray wolfCanis lupusArrived around 1949; after inbreeding nearly eliminated them, wolves were relocated to the island in 2018 and 2019.
Red foxVulpes vulpesThe most commonly seen predator, often around campgrounds.
American beaverCastor canadensisAn important engineer whose dams create wetlands and a key summer prey for wolves.
Snowshoe hareLepus americanusTurns white in winter and is a staple food for foxes.
Red squirrelTamiasciurus hudsonicusScolds hikers from conifers; one of the few rodents on the island.
North American river otterLontra canadensisFishes along shorelines and inland lakes.
American minkMustela visonA sleek predator of rocky shores and marshes.
American martenMartes americanaA tree-climbing weasel of the conifer forest.
ErmineMustela ermineaA small weasel that turns white in winter.
MuskratOndatra zibethicusBuilds lodges in marshy bays and beaver ponds.
Deer mousePeromyscus maniculatusA common small mammal of forests and campsites.
Little brown batMyotis lucifugusHunts insects over lakes and harbors on summer evenings.

Birds

Common loonGavia immerIts haunting calls drift across inland lakes on summer nights.
Bald eagleHaliaeetus leucocephalusNests along the shoreline and hunts fish in the harbors.
OspreyPandion haliaetusPlunges feet-first after fish in lakes and bays.
Herring gullLarus argentatusNests in colonies on small rocky islands.
Common merganserMergus merganserBroods of ducklings trail their mothers along rocky shores.
Pileated woodpeckerDryocopus pileatusChisels large rectangular holes in dead trees.
Gray jayPerisoreus canadensisA bold boreal bird that may visit campsites.
Boreal chickadeePoecile hudsonicaA brown-capped chickadee of spruce-fir forest.
Common ravenCorvus coraxOften follows wolves to scavenge their kills.
Black-throated green warblerDendroica virensA buzzy-voiced warbler of mixed conifer forest.
Swainson's thrushCatharus ustulatusIts spiraling flute-like song fills the evening forest.
White-throated sparrowZonotrichia albicollisSings its clear 'Oh sweet Canada' song from forest openings.
Sandhill craneGrus canadensisOccasionally seen in the island's wetlands.

Reptiles & amphibians

Painted turtleChrysemys pictaBasks on logs in inland lakes and ponds.
Common garter snakeThamnophis sirtalisThe most frequently seen of the island's few reptiles.
Red-bellied snakeStoreria occipitomaculataA small, secretive snake of the forest floor.
Wood frogRana sylvaticaCan survive being partly frozen in winter.
Mink frogRana septentrionalisA northern frog of lily-padded lakes that smells faintly of mink.
Spring peeperPseudacris cruciferIts chorus rings from wetlands soon after ice-out.
American toadBufo americanusCommon along trails and campsites.
Blue-spotted salamanderAmbystoma lateraleA dark, blue-flecked salamander of moist forest.

Fish

Lake troutSalvelinus namaycushThe cold waters around the island support several distinct forms of lake trout, from shallow-water leans to deepwater siscowets.
Brook troutSalvelinus fontinalisIncludes rare lake-dwelling 'coaster' brook trout that spawn along the island's shores.
CiscoCoregonus artediiA silvery forage fish of the lake and inland waters.
Lake whitefishCoregonus clupeaformisHistorically the basis of the island's commercial fishery.
Northern pikeEsox luciusA toothy predator of the inland lakes.
WalleyeSander vitreusFound in several of the island's inland lakes.
Yellow perchPerca flavescensCommon in weedy bays and inland lakes.
Mottled sculpinCottus bairdiiA bottom-dwelling fish of streams and rocky shallows.

Insects & other invertebrates

DragonhunterHagenius brevistylusA large clubtail dragonfly that preys on other dragonflies.
Chalk-fronted corporalLadona juliaAn abundant early-summer dragonfly of bog edges and trails.
Lake darnerAeshna eremitaA big northern darner patrolling lakeshores.

Species totals are the park's certified NPSpecies checklist (occurrence: present).

Climate & When to Visit

Lake Superior governs Isle Royale's weather. The lake stays cold all year, so summers on the island are cool and often foggy, with daytime highs typically in the 60s to low 70s°F near the shore and chilly nights. Interior ridges, farther from the water's influence, can be noticeably warmer on summer afternoons, which is one reason sugar maple forests grow inland while boreal conifers hug the coasts.

The lake also brings storms. Strong winds can raise dangerous waves in hours, cancelling ferry and floatplane trips and stranding paddlers. Fog is common in early summer when warm air passes over cold water. Winters are long, snowy, and severe, which is why the park closes from November 1 through mid-April.

Spring
Late and cold; the park reopens in mid-April, but ice, snowbanks, and near-freezing nights linger into May.
Summer
Cool and pleasant with frequent fog; biting insects peak in June and July, and the lake stays cold enough to cause hypothermia.
Autumn
Crisp days and brilliant birch and maple color in September, with increasing gales on the lake.
Winter
The park is closed; deep snow, subzero cold, and ice cover the island, occasionally bridging it to the mainland.

Weather hazards

Cold water: Lake Superior is cold enough to cause hypothermia within minutes, so wear a life jacket and stay close to shore when paddling.
Storms and high waves: Wind can build dangerous waves quickly; plan extra days in case boats and planes cannot travel.
Fog: Dense fog can roll in suddenly, disorienting boaters and delaying transportation.
Remoteness: Help can be hours or days away, so carry a first-aid kit, water treatment, and a plan for emergencies.
Moose encounters: Give moose at least 50 feet, especially cows with calves in spring and bulls in the autumn rut.

Isle Royale Questions

When is the best time to visit Isle Royale National Park?

The park is open from April 16 to October 31, with ferry service typically running from mid-May through September. July and August bring the warmest weather, while late August and September offer fewer bugs and fall color.

What is the weather like on Isle Royale in summer?

Expect cool, changeable conditions shaped by Lake Superior: highs usually in the 60s to low 70s°F, cold nights, and frequent fog. Pack rain gear and warm layers even in July.

Why does Isle Royale close in winter?

Severe cold, deep snow, ice, and storms on Lake Superior make safe travel to and from the island impossible. The park closes from November 1 through April 15, although researchers still visit in winter to study the wolves and moose.

Will I see wolves or moose on Isle Royale?

Moose are fairly common and many visitors see one, especially near inland lakes and wetlands at dawn and dusk. Wolves are elusive, and few visitors glimpse them, though tracks and scat are often found on trails.

How did Isle Royale form?

The island is made of lava flows and river gravels deposited about 1.1 billion years ago as the Midcontinent Rift tore at North America. The layers later tilted, and erosion and glaciers carved the hard lava into the island's ridges and the softer rock into valleys and harbors.

How do you get to Isle Royale?

By passenger ferry from Houghton or Copper Harbor in Michigan, or from Grand Portage in Minnesota, or by seaplane from Houghton. Private boats may also visit. There are no roads or vehicles on the island.

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Sources: www.nps.gov · en.wikipedia.org · en.wikipedia.org · carto.nationalmap.gov · irmaservices.nps.gov · Official NPS site