Activate Ashnod's Altar by sacrificing Spark Double, adding {C}{C}.
When Spark Double dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
As Spark Double enters the battlefield, choose to have it enter the battlefield as a copy of River Kelpie with a +1/+1 counter on it.
When Spark Double enters the battlefield, it and River Kelpie's first abilities trigger.
Resolve both triggers, causing you to draw two cards.
Repeat.

Infinite card draw

Infinite draw triggers

Near-infinite colorless mana

Near-infinite death triggers

Near-infinite creature ETB

Near-infinite creature LTB

Near-infinite creature sacrifice triggers

Spark Double

River Kelpie

Ashnod's Altar

Activate Altar of Dementia by sacrificing Moritte of the Frost.
When Moritte of the Frost dies, its persist ability triggers, returning it from your graveyard to the battlefield as a copy of River Kelpie with two additional +1/+1 counters on it and a -1/-1 counter on it.
Remove one +1/+1 counter and one -1/-1 counter from Moritte of the Frost as a state-based action.
When Moritte of the Frost enters the battlefield, it and River Kelpie's first ability triggers, causing you to draw two cards.
Resolve the Altar of Dementia ability from step 1, milling target player a number of cards equal to Moritte of the Frost's power at the time of activation.
Repeat.

Near-infinite mill

Infinite self-mill

Moritte of the Frost

Altar of Dementia

Activate Ashnod's Altar by sacrificing River Kelpie, adding {C}{C}.
River Kelpie dies, triggering its persist ability, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
River Kelpie enters the battlefield, triggering itself and Denry Klin.
Resolve the River Kelpie trigger, causing you to draw a card.
Resolve the Denry Klin trigger, putting a +1/+1 counter on River Kelpie.
Repeat.

Denry Klin, Editor in Chief

Activate Spawning Pit by sacrificing River Kelpie.
River Kelpie's persist triggers, returning it from your graveyard to the battlefield.
River Kelpie enters the battlefield, causing you to draw.
Resolve the Spawning Pit trigger, putting a charge counter on it.
Repeat.

Vizier of Remedies

Spawning Pit

Activate Spawning Pit by sacrificing River Kelpie.
River Kelpie's persist triggers, returning it from your graveyard to the battlefield.
River Kelpie enters the battlefield, causing you to draw.
Resolve the Spawning Pit ability.
Repeat.

Solemnity

Cast Selective Memory by paying {3}{U}.
Storm-Kiln Artist triggers, creating a Treasure token.
Resolve Selective Memory, exiling all nonland cards from your library.
Activate the Treasure token by tapping and sacrificing it, adding {R}.
Cast Flame Jab from your graveyard for its retrace cost by paying {R} and discarding a land card.
Storm-Kiln Artist and River Kelpie trigger.
Resolve the Storm-Kiln Artist trigger, creating a Treasure token.
Resolve the River Kelpie trigger, causing you to draw a land card.
Resolve Flame Jab, dealing 1 damage to any target.
Repeat from step 4.

Near-infinite damage

Near-infinite draw triggers

Near-infinite magecraft triggers

Near-infinite storm count

Selective Memory

Storm-Kiln Artist

Flame Jab

Cast Selective Memory by paying {3}{U}.
Birgi triggers, adding {R}.
Resolve Selective Memory, exiling all nonland cards from your library.
Cast Flame Jab from your graveyard for its retrace cost by paying {R} and discarding a land card.
Birgi and River Kelpie trigger.
Resolve the Birgi trigger, adding {R}.
Resolve the River Kelpie trigger, causing you to draw a land card.
Resolve Flame Jab, dealing 1 damage to any target.
Repeat from step 4.

Combos with "River Kelpie"