Ah Pah Dam

{{Infobox dam

| name = Ah Pah Dam

| name_official =

| image = Ah Pah Dam project, Klamath river, 1951.svg

| image_size =

| image_caption = Approximate extent of the planned reservoir

| image_alt =

| location_map =

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| coordinates =

| country = United States

| location = Humboldt County, Northern California

| status = Unbuilt

| construction_began =

| opening =

| demolished =

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| owner = U.S. Bureau of Reclamation

| dam_type = Concrete thick arch

| dam_height = {{convert|813|ft|m|abbr=on}}

| dam_length = {{convert|3500|ft|m|abbr=on}}

| dam_width_crest =

| dam_width_base =

| dam_volume =

| dam_crosses = Klamath River

| spillway_count =

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| res_name =

| res_capacity_total = {{convert|15000000|acre.ft|km3|abbr=on}}

| res_catchment = {{convert|14700|sqmi|km2|abbr=on}}

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| res_max_depth =

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| plant_turbines =

| plant_capacity = 900–1,700 MW

| plant_annual_gen =

| website =

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}}

Ah Pah Dam was a proposed dam on the Klamath River in the U.S. state of California proposed by the United States Bureau of Reclamation as part of its United Western Investigation study in 1951. It was to have been {{convert|813|ft|m}} high and was to be located {{convert|12|mi|km}} upstream of the river's mouth. It would have been taller than any existing dam in the United States and it would stand almost as tall as the Transamerica Pyramid building in San Francisco, but would have been much more massive. It would have flooded {{convert|40|mi|km}} of the Trinity River, including the Yurok, Karuk and Hupa Indian Reservations, the lower Salmon River, and {{convert|70|mi|km}} of the Klamath River, creating a reservoir with a volume of {{convert|15000000|acre.ft|km3}} – three fifths the size of Lake Mead, and over three times the size of the current largest reservoir in California, Shasta Lake. The water would flow by gravity through a tunnel {{convert|60|mi|km}} long to the Sacramento River just above Redding and onward to Southern California, in an extreme diversion plan known as the Klamath Diversion. The tunnel would have been located near the southernmost extent of the reservoir. It was named in the language of the Yurok people.